What Does SPF Stand For and What Does Sun Protection Factor Mean?

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

Why Does Light Bend When It Enters Water?

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

Why Does the Wet Spot On a Fabric Look Darker?

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

Why Do the Stagecoach Wheels Sometimes Turn Backwards In Western Movies?

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

Why Don’t Light Bulbs Last Forever or Longer Than They Do?

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

How Do Halogen Light Bulbs Work?

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

Why Are Primary Colors Blue, Green, and Red In Science But Blue, Yellow, and Red In Art Class?

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

What Is Black, Is Black a Color?

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

How Do Those Luminous Glowing Light Sticks Work?

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

Why Does a White Shirt Glow Brightly Under a Black Light?

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

Does Gravity Diminish At a Certain Distance From Earth?

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

How Do Big Airplanes Fly When They Are So Heavy?

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

Why Does the Lone Ranger Use Silver Bullets?

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

Why Do Guns Fire Spinning Bullets and What Is Angular Momentum?

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

Why Are Racing Car Tires So Smooth When They Need All the Traction They Can Get?

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

What Makes Things Happen and Where Does Entropy Come From?

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

How Can Energy Be Recycled To Save Resources Just Like Paper and Plastic?

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

What Does DNA Stand For and What Is DNA Made Of?

Those ladders of fuzzy black dashes used as evidence in court are just a way of making DNA visible to jurors and other ardent scholars of biochemical science certain things that are too small to see, even with a microscope. They’re the end result of a number of laboratory manipulations that never get explained in … Read more

What Is an Easy Way To Convert Celsius To Fahrenheit?

Yes, there is a much simpler way, and it’s a shame they don’t teach it in school. Once those complicated formulas with all their parentheses and 32s got into a textbook somewhere, they seem to have taken on a life of their own. Here’s the simple method: To convert a Celsius temperature to Fahrenheit, just … Read more

Would a BB Dropped From the Top of a Tall Building Kill Somebody If It Hit Them On the Head?

Pedestrians in the vicinity of Chicago’s 1454-foot Sears Tower need not fear. Hatted or not, they are in little danger from purely scientific experiments such as yours. (We won’t deign to discuss water balloons.) What you undoubtedly have in mind is the acceleration due to gravity, the fact that a falling object will fall faster … Read more

What Makes a Magnet Attract Iron, But Not Aluminum or Copper?

Magnets are attracted only to other magnets. A piece of iron contains billions of tiny magnets, but copper and aluminum don’t. The only thing that the pole of a magnet will attract is the opposite pole of another magnet. It’s exactly the same as with electric charges: The only thing that a positive electric charge … Read more

Why Are Cucumbers Cool and Are They Really Twenty Degrees Cooler Than Their Surroundings?

Twenty degrees, eh? Well, let’s just see about that. (We’ll assume that we’re dealing with Fahrenheit cucumbers, rather than Celsius.) If cucumbers are always twenty degrees cooler than their surroundings, let’s put a cucumber into a barrel with a whole bunch of other cucumbers and wait to see what happens. Will they fight it out, … Read more

Why Can’t Superman See Through Lead With His X-Ray Vision?

superman x-ray vision

Superman could probably see through lead with his X-ray vision if he really tried. It’s just that his inventors, Jerry Siegel and Joe Shuster, told him that he can see through anything but lead, and like any good cartoon character, he faithfully obeys his creators. Siegel and Shuster’s idea seems to have been that X-rays … Read more

Why Does Water Put Out a Fire and How?

Before we get any further, note well: Water must never be used on an electrical fire or on an oil or grease fire. Reasons: Water conducts electricity and can lead it elsewhere, perhaps to your very own feet. And because water won’t mix with oil or grease, it just scrambles it around and spreads the … Read more

What Makes Ice So Slippery?

Solid ice itself isn’t slippery. There’s a thin film of liquid water on its surface that the skaters are sliding on. Solids in general aren’t slippery because their surface molecules are tied tightly together and can’t roll around like ball bearings. The molecules of liquids, on the other hand, are free to move around, so … Read more

How Does the Boiling Temperature of Water Depend On the Weather?

The weather has only a small effect on the boiling temperature of water. When people go around saying that water boils at 212 degrees Fahrenheit (100 degrees Celsius) at sea level, they’re speaking rather loosely. The standard definition of the boiling temperature of pure water says nothing about sea level. It is defined in terms … Read more

How Does Water Seek its Own Level and Why?

“Water seeks its own level” is a catch phrase that was probably uttered by a Greek philosopher two thousand years ago, and people have been parroting it ever since. In plain language, it means that water will lie flat whenever it can. If a body of water, anywhere from a bucket to a bathtub to … Read more

How Does Hot Water Freeze Faster Than Cold Water?

This controversy has been raging ever since the early seventeenth century, when Sir Francis Bacon became a charter member of the Betcha-the-hot-water-freezes-first camp. The only appropriate answer to this puzzle is, “It depends.” It depends on precisely how the freezing is being carried out. Freezing water may sound like the simplest of happenings, but there … Read more

Why Are Soap Bubbles Round and Not Square?

Let’s put it this way: You’d be pretty surprised if they were square, wouldn’t you? That’s because all of our experience since we were babies tells us that Mother Nature prefers smoothness. There just aren’t many natural objects that have! sharp points or jangling angles. The major exception is certain mineral crystals, which occur in … Read more

Can Fish Get the Bends From Staying Underwater For Too Long?

Fortunately, it isn’t necessary to answer that question, because divers, and fish,  don’t get the bends (more accurately known as decompression sickness) from staying down too long. Divers get the bends by coming up too fast, but fish can indeed get the bends from other causes. When the water pressure on a diver’s body is … Read more

How Does a Fish Swim Up and Down In Water?

Of course, it can always swish its tail and swim to wherever it wants to go, but that’s just a temporary solution. What it would really like to do is adapt its body to the pressure of the new depth, so that it can maintain its neutral buoyancy and rest there without constantly having to … Read more

How Do Submarines Change Their Buoyancy To Sink and Float?

Very simply. They change their amount of internal air space, thereby changing their density. You want to dive? You let water into your ballast tanks. You want to surface? You blow the water out with compressed air. It gets a bit tricky in reality, though, because the density of seawater actually varies a bit, depending … Read more

Why Do Ships Float On Water and Why Do Heavy Things Sink?

The pat answer to the everyday puzzle of why things float invariably goes like this: “According to Archimedes’ principle, a body immersed in a fluid is buoyed up by a force equal to the weight of the fluid displaced. And that’s why things float.” Perfectly correct, of course, but just about as illuminating as a … Read more

How Does Perspiration and Evaporation Help Cool Us Down?

The answer is, “It does and it doesn’t.” Maybe that’s why people go around simply parroting the prefabricated, though hardly enlightening, answer, that “evaporation is a cooling process.” We notice that our sweat glands are exuding a liquid water containing a little salt and urea, onto our skin only at certain times, such as (a) … Read more

How Do They Make All Those Colors In Neon Signs?

The colors are actually glowing atoms, stimulated by electricity. It’s pretty much the same as making the colors in fireworks: Stimulate atoms with energy, and they’ll quickly get rid of the excess energy by emitting light of their own characteristic colors. There are a couple of differences (fortunately) between fireworks and neon signs. In neon … Read more

How Do They Make All Those Colors In Fireworks?

They add chemicals to the explosive mixtures that emit specific colors of light when subjected to heat. You could throw some of these same chemicals into your fireplace if you thought that a green fire, for example, might be more romantic. When you throw an atom into a fire, it can pick up some of … Read more

What Makes a Snowball Hold Together?

It’s a nice idea, because snowflakes certainly do have beautifully complex shapes, with spikes, lacy edges, and all the rest. But interlocking hooks and loops are a bit too much to expect. Besides, they’re much too fragile and brittle; when you pack them together they suffer a crushing experience. The answer lies in the fact … Read more

How Do Snowmaking Machines Work?

Just pumping a spray of water into the air wouldn’t work very well, except perhaps in extremely cold weather. And by the way, the machines don’t produce actual snowflakes; they make tiny beads of ice, each one around ten thousandths of an inch in diameter. The simple spraying of water wouldn’t work because when water … Read more

How are Instant Coffee and Freeze Dried Coffee Made?

Freeze dried coffee is made by the sublimation of ice. Freeze-dried coffee differs from ordinary instant coffee in an important way. To make either kind of fast-beverage powder, they first brew two-thousand-pound batches of incredibly strong coffee. If they are making instant coffee, they then quick-dry this thick brew by dropping it down through a … Read more

Can Snow Evaporate and What is Sublimation?

The snow in winter isn’t melting if it’s below freezing; it is actually going straight off into the air as water vapor, without having to melt into liquid water first. We might be tempted to say that the snow is evaporating, but scientists prefer to reserve the word “evaporation” for liquids only. So when a … Read more

How Does the Greenhouse Effect Cause Global Warming?

The Greenhouse Effect is the effect of infrared radiation-trapping by the Earth’s atmosphere, which can raise the average temperature at the surface of the entire globe, just as the trapping of infrared radiation within a greenhouse raises the temperature inside. The overall temperature of the Earth’s surface, averaged over all seasons and climates, depends on … Read more

Why Is a Greenhouse Warm and How Does a Greenhouse Trap Heat?

Greenhouses, sometimes called hothouses or glasshouses, are always naturally warmer, without any artificial heating. But believe it or not, the main reason is not what everybody refers to as “the greenhouse effect.” A greenhouse is just a closed, glass container for plants. The glass lets in sunlight, which the plants need for growth, while keeping … Read more

How Can You Tell the Temperature By Listening To Crickets?

You can tell the temperature by counting the chirps crickets make. All cold-blooded animals perform their functions faster at higher temperatures. Just compare how fast the ants run around in cool and hot weather. Crickets are no exception. They chirp at a rate that is geared directly to the temperature. To understand their message, all … Read more

Who Invented the First Barometer and Why Is Air Pressure Measured in Inches?

First of all, please don’t call air pressure “barometric pressure.” The air around us has a temperature that is measured by a thermometer, a humidity that is measured by a hygrometer, and a pressure that is measured by a barometer. Television weather reporters wouldn’t dream of talking about the air’s “thermometric temperature” or its “hygroscopic … Read more

Why Can We See Through Air and Why is Chlorine Gas Green?

It’s very simple. The molecules in air are so far apart that we’re actually looking through empty space. To notice anything at all, we would have to be able to see the individual molecules, but air molecules are about a thousand times smaller than anything we can observe, even with a microscope. We’re talking about … Read more

Why Do Meteorologists Report Temperature In the Shade But Not Temperature in the Sun?

While the temperature “in the shade” is a fairly reproducible figure, the temperature “in the sun” depends too much on whose temperature you’re talking about. Different objects, including different people in differed clothing, will experience different temperatures in the sun because they will absorb different amounts of different portions of the sunlight’s spectrum. Light-colored clothing, … Read more

Why is it Colder In the Winter Than In the Summer?

Right on. When it is winter on the part of the Earth where you live (northern or southern hemisphere), your hemisphere is leaning away from the sun a bit. That is, the axis of the Earth wobbles, so that during winter in the northern hemisphere the North Pole is farther from the sun than the … Read more

Why is the Risk of Sunburn is Greatest Between 10am and 2pm?

The ninety-three-million-mile separation between the sun and the Earth pays little attention to our lunchtime or recreational schedule. The sun is essentially the same distance from your rapidly reddening nose at all times of day. But the strength of the sunshine varies, for two reasons: one atmospheric and one geometric. Picture the Earth as a … Read more

Why do Waves Always Break Parallel To the Shore?

Waves can tell when they’re approaching a shore and actually turn to line up with it. What makes waves, of course, is wind blowing across the water’s surface. But it can’t be that the wind is always blowing the waves straight in to shore. Out in the ocean, the wind may be blowing every which … Read more