Diagram of a carbon atom
When a star has changed all of the hydrogen atoms into helium, it begins to convert the helium atoms into carbon atoms and oxygen atoms. All of the carbon in the universe was made inside stars. Carbon atoms are heavier than helium or hydrogen, because they have six protons and six neutrons in the nucleus, and six electrons going around the outside.
The six electrons can't all go around the nucleus at the same distance - only two electrons can fit in the inner shell. So some of the electrons have to go farther away from the nucleus. A carbon atom has two shells, with two electrons in the inner shell and four electrons in the outer shell.
But that outer shell could hold as many as eight electrons - it's not full. That makes it very easy for carbon to combine with other atoms to make bigger molecules. A lot of carbon combines with oxygen to make carbon monoxide (one oxygen atom) or carbon dioxide (two oxygen atoms), for example.
Carbon soot coming out of a truck
All living things on Earth are made mostly of hydro-carbons (molecules of hydrogen and carbon) and water (molecules of hydrogen and oxygen). Both plants and animals are about 18 per cent carbon. We're all made out of the insides of stars! Plants get carbon by taking carbon dioxide out of the air and breaking off the oxygen atoms, and animals (including people) get carbon by eating plants or other animals. Animals recombine the carbon with oxygen to make carbon dioxide, which is what you breathe out. And when plants and animals die, their bodies also gradually become carbon dioxide again.
Most of what we use for fuel is made of carbon, too. Coal, gasoline, oil, and wood are all made of hydro-carbons. In the picture you can see leftover carbon dust pouring out of this truck that burns diesel fuel. It's this carbon pouring into the air that's causing global warming.
To find out more about carbon atoms, check out these books from Amazon.com or from your library: