All living things on earth, plants or animals, have one thing in common they are all made up of cells. Your entire body, your bones, your muscles, your skin, your blood, your teeth, your nerves, your hair, contains more than 10 million million (10,000,000,000,000) cells! Most of these cells are so tiny that you can see them only under a microscope. For example, your red blood cells are so tiny that 5,000,000 of them would fit inside a drop of blood the size of this letter a.
Different parts of your body are made up of different kinds of cells, and each kind does a special job that no other kind of cell can do. For example, muscle cells in your eyeballs move your eyes across this page you are reading, than nerve cells in your eyes send messages to your brain of what you have read. Red blood cells carry oxygen from your lungs to the rest of your body to help them grow as you are reading, while white blood cells are busy fighting off the bacteria that could make you too sick to go on reading.
Some cells are so tiny that even the most modern electron microscope, which magnifies 200,000 times and can enlarge an ant to a length of mile, cannot show the details of these tiny cells!