Karen Hill

The cloudiness is a mass of tiny air bubbles, air that was dissolved in the water and expelled when the water froze. You can see the individual bubbles through a magnifying glass. There is always some air dissolved in any water that has been exposed to, well, the air. For this, the world’s fish are … Read more

Absolutely nothing would happen, as far as you would be able to tell. The ice and the water would dwell in peaceful coexistence. But down at the molecular level, a chaotic dance would be going on. Zero degrees Celsius (32 degrees Fahrenheit) is indeed both the freezing point of liquid water and the melting point … Read more

Liquids do get somewhat thicker as they are cooled, and because all glass was once a liquid while it was hot and being formed into shape, some people like to think that it gets thicker and thicker as it “supercooled,” until it gets so thick that it acts like a solid. Well, the truth is … Read more

Detergents aren’t soap, although soap is a detergent. The word “detergent” simply means a cleansing substance, from the Latin detergere, to wipe off. After more than two thousand years of using soap, which is easy to make by boiling up wood ashes with animal fat (don’t you wonder how that discovery was made?), humans finally … Read more

The stuff in the dryer sheet happens to be good at both jobs. You can obtain the static-elimination function all by itself as a liquid in a spray can, so you can de-static your clothing even while you’re wearing it without your having to, DO NOT TRY THIS AT HOME!, climb into the dryer. The … Read more

You’re lucky you asked that question today, because today is bargain day. I’m going to give you four answers for the price of one. It’s not because I’m feeling generous, but because I can’t make up my own mind about which one to believe. To my knowledge, the National Science Foundation has not yet funded … Read more

For years, people have observed this phenomenon without giving it a second, or even a first, thought. Hundreds of less-than-fastidious, coffee sipping scientists have probably glanced at the ring, mumbled something about surface tension and told their lab assistants to clean it up. But it wasn’t until 1997 that six scientists at the University of … Read more

That’s not why the Panama Canal has locks. The locks are there for the purpose of lifting ships over the hump of land known as the Isthmus of Panama. To get over that hump, the ships have to be floated upward 85 feet (26 meters) above the entrance ocean and then lowered back down into … Read more

When you say “salty,” you’re undoubtedly thinking of sodium chloride, common table salt. But to a chemist, a salt is any member of a large class of chemicals, and there are dozens of them in the oceans. To put the word “salt” in perspective, please indulge me in a one-paragraph chemistry lesson. A “molecule” of … Read more

That’s a common belief that just doesn’t hold water, so to speak. First of all, the ocean’s surface isn’t exactly what you’d call a mirror. And second, how come it’s a much darker blue than the sky? No, the world’s oceans really and truly are blue, many different shades of blue (ask any sailor), depending … Read more

Water the one substance that is indispensable to all living things. Water makes up more than half of our own body weights. Water is the most abundant chemical on Earth, with more than a billion billion tons of it covering 71 percent of the planet’s surface and probably another billion tons in those little plastic … Read more

It isn’t. Satellites and space shuttles do indeed get cold up there, but it’s not because it’s cold up there. First of all, there’s really no such thing as cold, no matter what the penguins tell you. Cold is a linguistic concept, not a scientific one. Our caveperson ancestors needed a word for “not hot,” … Read more

Yes, but only once in a great while. There has to be exactly the right kind of smoke or dust in the air. It happened most spectacularly in 1883, when the Indonesian volcano Krakatau blew its top, spewing dust all around the globe. The bluest moon since Krakatau was caused by a series of forest … Read more

There are two answers: No and Yes. The No answer: The expression “when the moon turns blue” was used for hundreds of years to mean “when hell freezes over” or “fat chance.” “Blue moon” first appeared in print in the nineteenth century, but was probably used even before that because it’s a quirky idea and … Read more

It’s easy to fool yourself into thinking that the moon is bigger when it’s full, and that it therefore pulls on the oceans more strongly to make higher tides. But the moon is always the same size and distance away as it circles Earth. It is just lit up differently by the sun at different … Read more

Whenever someone pompously proclaims that the ocean’s tides are caused by the moon, everybody mutters, “Uh, okay,” and goes away just as puzzled as before. “It’s the moon” is a cop-out, because a real explanation requires a lot more than that. Ocean tides are the net result of several forces produced by motions of the … Read more

Sounds odd, doesn’t it? Either it’s the most colossal coincidence that ever occurred, or there’s something real fishy going on. Well, even the fishiest-seeming coincidences can have rational explanations. Your first guess might be that the moon isn’t spinning on its axis the way Earth is, and that it just goes around us, maintaining the … Read more

For much the same reason that stars twinkle, except that there are enough light rays coming from the object that no matter how much they scatter, some of them will always be reaching your eyes. So there’s no actual twinkling. When you look down the road on a hot day, you may see shimmering “lines … Read more

The answer that you see everywhere is that the twinkling is caused by turbulence in the atmosphere, which distorts the light coming from the star. But that doesn’t explain why “atmospheric turbulence,” whatever that is, should distort light in the first place, or where the on-and-off blinking effect comes from, or why only stars, but … Read more

Practically everybody has noticed this oddity at one time or another. When the moon is low, near the horizon, it looks huge compared with how it looks a few hours later when it is higher overhead. The effect is especially noticeable when it is a big, beautiful, full disk, a full moon. But you can … Read more

It depends on how far you are from the lightning. The closer you are, the higher the pitch of the sound you hear; the farther away you are, the lower the rumble. First, we have to remind ourselves of what thunder is. A stroke of lightning is extremely fast; it occurs with what might be … Read more

There’s a lot of nonsense out there about sonic booms. The Columbia Encyclopedia 5th edition (1993) says, “An object such as an airplane generates sound. When the speed of the object reaches or exceeds the speed of sound, the object catches up with its own noise” (I wish some politicians would do that), which causes … Read more

The crack of a bullwhip is actually a miniature sonic boom, produced because the tip of the whip is traveling through the air faster than the speed of sound. When the cat master snaps his whip sharply, he’s putting a great deal of energy into the handle end. That energy has no place to go … Read more

You’d get an exceedingly clean vacuum. But seriously, I don’t know why you’d want to imagine a thing like that, because there is nothing cleaner than a true vacuum; it is the epitome of nothingness. I’ll assume, however, that you ask the question out of scientific curiosity, rather than because it’s funny. What is a … Read more

Sorry, but yes. Not actual fragments, but individual molecules, molecules that have evaporated from the “stuff” and have floated through the air to your nose. But don’t get sick at the thought. It takes only an incredibly small number of molecules to be detected by humans as an odor. And the molecules aren’t even molecules … Read more

Every radioactive, or unstable, atom has a certain probability of disintegrating within a certain period of time. Some kinds of radioactive atoms are more unstable than others and have higher probabilities of disintegrating. We can’t tell when any single atom will disintegrate, but averaged over the zillions of atoms in even a minute speck of … Read more

Radiocarbon dating won’t help you to determine the age of anything that is still alive, such as a twelve-year-old posing as a twenty-five-year-old in an Internet chat room. It’s is useful for determining the ages of plant or animal matter that died anywhere from around five hundred to fifty thousand years ago. Ever since its … Read more

Probably not, if you mean making electricity from solar panels. There certainly is lots of sunshine, but capturing it and converting it efficiently is the problem. Let’s do the arithmetic. Every day, the sun shines down upon the surface of Earth an amount of energy equal to three times the world’s annual energy consumption. That … Read more

Absolutely. And during the autumnal equinox as well. And on Tuesdays in February, and anytime during the fourth game of the World Series when the count is three and two on a left-handed batter. Get the picture? The point, of course, is that equinoxes have nothing whatsoever to do with balancing eggs. But old superstitions … Read more

It’s just another one of those urban legends, probably started by an overenthusiastic physics teacher. But it’s based upon a grain of truth. Moving fluids such as air and water are slightly affected by Earth’s rotation. The phenomenon is called the Coriolis effect, after the French mathematician Gustave Gaspard Coriolis (1792–1843), who first realized that … Read more

Boy, you must really want to lose weight. Yes, you’d weigh very slightly less. You’re probably thinking that your weight is a consequence of Earth’s gravitational pull on your body, and that if you’re below the surface gravity has already done part of its job, so there is slightly less pull left. Well, there is … Read more

The riddle part is that polar bears live at the North Pole, not the South Pole. Furthermore, a polar bear at the equator wouldn’t be a polar bear, would he? He’d be an equatorial bear. But let’s take the question at face value; namely, will a bear, or anything else, for that matter, weigh less … Read more

No, even though they’re a lot higher and moving a lot faster than an airplane, the situation is still the same. To them, Earth’s surface appears to be “moving backward” at their speed of 18,000 miles per hour (29,000 kilometers per hour), just as it appears to you when you’re in a car or an … Read more

Because even when you’re flying off to a remote island to get away from it all, you can’t escape being part of “it all.” Your airplane is attached to Earth almost as tightly as the mountains below are, except that the airplane is (we hope) at a higher altitude. Your pilot would be the first … Read more

No, it’s because Earth’s rotation is a uniform, unvarying motion, and we can feel only changes in motion (Techspeak: acceleration). Any time a moving object is diverted from its motion, either by a change in its direction or a change in its speed, we say that it has experienced an acceleration. Acceleration doesn’t just mean … Read more

The South Pole, where the average temperature is about 56 degrees below zero degrees Fahrenheit (−49 degrees Celsius). At the North Pole the average temperature is a relatively balmy 20 degrees below zero (−29 degrees Celsius). Antarctica is actually a continent, with the ice and snow lying on top of a huge land mass, whereas … Read more

The air does keep getting colder, by an average of about 3.6 degrees Fahrenheit for every thousand feet (6.5 degrees Celsius per kilometer) , up to around 33,000 feet (10,000 meters) above sea level. That’s just a bit higher than the cruising altitude of large commercial jet aircraft. You may have heard the airliner’s captain … Read more

Obviously, because it’s always colder up there. But why is it always colder up in the mountains than down at the seashore? After all, doesn’t hot air rise? Shouldn’t it therefore be hotter up there? There’s certainly plenty of hot air in equatorial Tanzania, but Kilimanjaro, which thrusts its peak 19, 340 feet (5,895 meters) … Read more

It’s pretty hard to look inside that steel tank and see how much propane is left before you fire up the grill, isn’t it? Not all grills have pressure gauges. But hardware stores sell an ingenious little indicator that looks like a strip of plastic because it is. You stick it onto the outside of … Read more

People say heat rises because they’re speaking carelessly. The statement is just a lot of hot air, because heat doesn’t rise. What they mean to say is that hot air rises. Heat is one of many forms of energy; it is energy in the form of moving molecules. But it’s meaningless to say that any … Read more

Because the center of the planet is the center of Earth’s gravity: its center of gravity. You’ve heard the expression “center of gravity” before, and now’s your chance to understand what it really means. But first, what is gravity, or, more properly, gravitation? Gravitation is one of the three fundamental forces in Nature. (The other … Read more

This question is as old as electric power itself. It has been asked almost as often as “Do you love me?” and with equally unconvincing replies. The common answer, “The birds aren’t electrocuted because they’re not grounded”, doesn’t get to the root of the question. Does everyone who walks away after that explanation really know … Read more

Electric current is measured in amperes. An ampere is a huge unit of electric current, equivalent to 6 billion billion (6 followed by 18 zeros) electrons passing by every second. So you often hear talk of milliamperes or milliamps,  thousandths of amperes. One milliamp passing through your body will cause a mild tingling sensation. Ten … Read more

Voltage in itself isn’t dangerous. A 10,000–volt shock can be no more disturbing than a pinprick, but you can get a serious jolt from a 12–volt automobile battery. What’s dangerous is the amount of electric current that flows through your body as a result of the voltage. A current of electricity, as you undoubtedly know, … Read more

Unfortunately, no, for several reasons. First of all, the world’s supply of air conditioners and refrigerators isn’t anywhere near what you might think by looking around your neighborhood. But even if every citizen of the less-developed nations were privileged to enjoy cool bedrooms and frozen pizzas, the amount of available coolth wouldn’t amount to an … Read more

Light a match and, while it’s burning, twist it into a variety of positions. The flame keeps pointing unerringly upward, regardless of the orientation of its fuel. How, indeed, does it “know”? You are well aware that hot air rises. A flame, whatever it is, must therefore be carried upward by the rising current of … Read more

Yes, there is a hottest possible temperature. But let’s start off at merely warm and gradually turn up the heat. Heat is the energy that a substance contains within itself, due to the fact that its atoms and molecules are moving. But temperature is a man-made concept, invented so that we can converse among ourselves … Read more

They are indeed strange numbers for such common, everyday goings-on as the freezing and boiling of water. We’re stuck with them because a German glassblower and amateur physicist named Gabriel Fahrenheit (1686–1736) made a couple of bad decisions. Gadgets for measuring temperature had existed since about 1592, even though nobody knew what temperature was, and … Read more

Assuming that you haven’t forgotten to pay your gas bill, it’s because porcelain tile conducts heat better than that cozy bath mat does, even though they’re at the same temperature. It’s a common experience that certain things feel colder than others. People talk about “cold steel” as if the blade of a sword were somehow … Read more

Heat is energy. What kind of energy? It’s not electrical energy or nuclear energy or the kind of energy that your car has as you barrel down the highway. It’s the energy that an object contains within itself, because the particles that it’s made of, its atoms and molecules, are actually vibrating and bouncing around … Read more

If Tucson was twice as hot as Miami, it certainly wouldn’t be 160 degrees Fahrenheit (71 degrees Celsius). But that’s not because 160 degrees is too hot; it’s not hot enough. The temperature that is “twice as hot” as 80 degrees Fahrenheit, believe it or not, is 621 degrees Fahrenheit! Here’s what’s going on. First … Read more

Everything is hot. That is, it contains some heat. And as a consequence, it has a temperature. Even an ice cube contains heat. “Hot” is strictly a relative term. Heat is the ultimate form of energy, the form into which all other forms ultimately degenerate. There is energy of motion (Techspeak: kinetic energy), there is … Read more

Your question may sound silly to those who haven’t heard about it before, but chomping on those little candies really does make flashes of light. It may not help you at all to know that the phenomenon is called triboluminescence, but there, I’ve said it and done my duty as a scientist. Life Savers, it … Read more

Mirrors are the best reflectors of light that human ingenuity has been able to devise. Notice, however, that the light is reflected only from the backing of the mirror, after passing through the front layer of transparent glass. What is there about the backing that makes it such a good reflector? It’s a thin, smooth … Read more

Well, what does “transparent” mean? It means that any light being reflected in our direction from an object outside a glass window, for example, can pass right through the glass unobstructed and come out the other side, where our eyes can deal with it. We therefore see the object through the window. That’s why people … Read more

A radiometer (also known as a light mill, light windmill, or roentgenometer) is generally supposed to illustrate that light has pressure. But they don’t. If a machine could be a con artist, this gadget would take the cake. You’ve seen them. They look like a light bulb on a stand. Inside the bulb, which has … Read more

The SPF numbers aren’t sun -filtering factors, they’re sun -protecting factors. SPF stands for “sun protection factor.” The numbers are not telling you how much radiation they block out, but how much time you can spend in the sun before your skin turns red, a condition doctors call erythema. And that’s quite another matter. With … Read more

Whenever a scientist has to explain something about light, he or she has the choice of explaining it on the basis of light waves or light particles (Techspeak: photons), because light behaves as if it were both or either a particle and/or a wave. Explaining refraction on the basis of light’s being a wave would … Read more

I’ll assume that you’re in the dining room, concerned about soup on your necktie, although you may have noticed this phenomenon in other rooms under different circumstances. We see an object because light is coming from that object and entering our eyes. The more light coming from the object, the brighter it appears. And of … Read more

Photographic film wasn’t as sensitive as it is today, so the exposures had to be longer and therefore further apart in time. The cameras shot only 16 pictures per second, rather than 24. In that longer amount of time between pictures, the people moved farther, so in a second’s worth of pictures they seem to … Read more

This is the only remaining artificiality in today’s remarkable, computer-driven movie effects, which can make anything imaginable look real, no matter how bizarre, except, ironically, an old-fashioned stagecoach wheel. You can also see the effect with automobile wheels, in those television commercials that show the cars speeding along an open road. If you watch carefully, … Read more

The distance from your eyes to the mirror is irrelevant. It’s the distance from your eyes to a given object that counts, just as it would when you look at it without the mirror. The light reflected from an object has to get to your eyes somehow, or else you wouldn’t see it. The light … Read more

This is one of those loaded questions that can drive you crazy because the question itself is misleading. It starts with a mistaken assertion and asks us to carry on our reasoning from that point. But you can’t pursue the road to truth if somebody starts you off in the wrong direction. A mirror does … Read more

Lightbulbs are very carefully engineered to last for a certain length of time. A suspicious person might be tempted to say that they are carefully engineered to burn out after a certain length of time. There is no reason that a lightbulb couldn’t be designed to last almost indefinitely. But you probably wouldn’t like it. … Read more

Halogen lightbulbs contain a gas called a halogen, which makes them brighter, whiter, more efficient and longer-lasting. And, of course, much more expensive. A halogen lamp is a variation on the standard incandescent, as opposed to fluorescent, lamp. An incandescent lamp contains a tungsten filament enclosed in a glass bulb filled with gas. An electric … Read more

You can buy the common, 48-inch shop-light tube in home centers for a couple of dollars, whereas a small, thin fluorescent tube for under the kitchen cabinet might cost up to five times that amount. The answer is that the four-footers, used by the thousands in schools, factories and office buildings, vastly outsell the smaller, … Read more

Fluorescent lights were invented for one purpose: to confuse you. I’m glad to see that they’re doing their job. When an ordinary incandescent lightbulb burns out you can just screw in a new one with the help of a certain number of friends, depending on your vocation or ethnicity. But when a fluorescent light burns … Read more

Because they think of color differently. Scientists describe objectively what Nature provides. They therefore think of color as a fundamental characteristic of light itself. To a scientist, light of different colors is radiation of different wavelengths. Artists, on the other hand, create their own interpretations of Nature. They therefore tend to think of color subjectively, … Read more

A black surface is one whose molecules are absorbing all visible wavelengths of the light that is falling upon it, and reflecting virtually none of it back. So black isn’t really a color, because we define a color in terms of the specific combination of light wavelengths that reflect back into our eyes. But, of … Read more

First, we have to look at what “white” is. You’ve heard people say dozens of times that white light is the presence of all colors. But other people tell you that white isn’t a color at all, that it’s the absence of color. You use bleach to remove all color from your laundry and make … Read more

You mean those plastic rods full of liquid chemicals that are made by Omniglow and other companies and are sold at street fairs, festivals and concerts and that start glowing with green, yellow or blue light when you bend them, and that gradually lose their light after an hour or so? Never heard of them. … Read more

It’s the same fluorescence phenomenon as the Day-Glo colors. Most laundry detergents contain “brighteners” that absorb ultraviolet radiation from daylight and re-emit the energy as a bluish light that makes the shirt look “whiter and brighter.” Moreover, the blue cancels out any yellowish cast. When stimulated by an ultraviolet lamp, which is even richer in … Read more

In a Day-Glo colored object there’s a chemical that takes invisible ultraviolet radiation out of the daylight and converts it into visible light of the same color as the object. Thus, the object is not only reflecting its normal amount of colored light, but is actively emitting some light of the same color, which makes … Read more

This is purely an exercise in “What if?” Automobiles, of course, aren’t built sleek enough or strong enough to exceed the speed of sound or to withstand the physical stresses of the sound barrier. But it’s fun to think about. The answer is simple: Yes. Or, I could have posed a different question that would … Read more

It’s as if the lake magnifies the sound somehow, isn’t it? But it isn’t actually magnifying the sound, as a microphone and amplifying system would do; it’s just that more of the sound is being funneled toward your ears. Sound consists of vibrations of the air. The guy on the other side of the lake … Read more

Astronauts are not weightless in orbit. There’s a completely different reason why astronauts can do all those silly tricks for the cameras, such as performing somersaults in midair or sitting upside down on absolutely nothing, looking more witless than weightless. Earth’s gravitational attraction, like all gravitational attraction, reaches out indefinitely; it keeps getting weaker and … Read more

It can be done to wow the crowd at an air show, but it wouldn’t work for a commuter flight to Schenectady because, although it’s theoretically possible, passenger planes aren’t built to stand the stress. (Nor are the passengers.) A conventional airplane’s wings are curved or humped on top, and that produces lift for reasons … Read more

Even though I know something about how airplane flight works (and you will too, soon), it never ceases to amaze me. I remember landing after a transatlantic flight in a Boeing 747 and being directed by the crew to deplane directly onto the ground and into a waiting bus, instead of through one of those … Read more

Whether we realize it or not, we judge the motion of an airplane in the sky by its relation to common things on the ground, such as trees, telephone poles and houses. That’s the only way motion can be detected: in relation to something else. There’s no such thing as absolute motion; it’s all relative … Read more

Silver bullets serve mostly as a calling card, but they do have a very slight advantage over lead. Ordinary bullets are made of lead because lead is so heavy, or dense. And it’s cheap. We want a bullet to be as heavy as possible because we want it to have as much damage-causing energy as … Read more

A spinning bullet flies farther and truer than it would without the spin. And if your favorite sport is football rather than shooting, just about everything I’m going to say about spinning bullets also goes for spiraling passes. The fact that a spinning bullet or football goes farther may sound strange, because you’d think that … Read more

Quite dangerous. As we’ll see, physics tells us that when it hits the ground the bullet will have the same velocity it had when it left the muzzle of the pistol, which can be 700 to 800 miles per hour (1,100 to 1,300 kilometers per hour). But that ignores air resistance. More realistically, the bullet’s … Read more

That’s precisely why they’re smooth. Regular tires waste a lot of their potential road-grabbing surface by having grooves, which act like gullies to channel out rain and mud. But racing cars usually compete in good weather, so the rain-and-mud grooves aren’t necessary. They’re just wasted space that can better be used to add more road-grabbing … Read more

It has been rubbed off, and no, that’s not why they call it rubber, onto the road, whence it was scattered in the form of fine dust into that vast, complex everywhere that we call the environment. Some of it was then washed off the road and into sewers by rain, or else it was … Read more

Ho hum. I don’t know how many times this question has flashed into the minds of worrywarts in elevators, or how many times it has been asked of every friendly neighborhood physicist. It is easy to answer in one word (No), but thinking about it does raise a whole bunch of fun questions. First, here’s … Read more

No, but it sure would create a windfall for Chinese podiatrists. I suppose that everybody picks on China when they ask this question because China is the most populous country on Earth, containing 2.5 billion potentially sore feet. There are really two questions here, aside from the question of why people who ask this question … Read more

Ramps and loops on freeways enhance the traffic flow, from construction companies to politicians’ campaign chests. Sorry. They allow us to make left turns without getting killed by oncoming traffic. It’s a matter of simple geometry. When freeways and superhighways began to be built, engineers had to figure out how to allow traffic to make … Read more

There’s no such thing as a dumb question. Actually, yours is perhaps the most profound question in all of science. Nevertheless, it does have a fairly simple answer,  ever since a genius by the name of Josiah Willard Gibbs figured it all out in the late nineteenth century. The answer is that everywhere in nature … Read more

If by recycling you mean transforming something into a more useful form. We do it all the time. Power plants transform water, coal, or nuclear energy into electricity. In our kitchen toasters we transform electrical energy into heat energy. In our automobile engines we transform chemical energy into motion (kinetic energy). The different forms of … Read more