Although atoms are made up of even smaller pieces, the pieces that make them up are extremely rarely found by themselves, and in fact they are never found freely on earth, except in the laboratory or in fleeting instants before they reform into atoms.

Most atoms spend their time joined to other atoms to make molecules. Many common substances are molecules: water, ethyl alcohol, and salt, for example. Workers are molecules -- very big ones -- as is DNA. Everything that isn't a molecule -- air, wood, people, the earth -- is a mixture of molecules.

Atoms come in many types, called elements. Ninety-one elements occur in nature, and people have created about twenty others artificially. Living things are made predominantly of about ten elements: carbon, hydrogen, oxygen, phosphorous, potassium, iodine, nitrogen, sulfur, calcium, and iron. Living things contain smaller amounts of other elements.

The elements that make up living things are not found in molecules consisting of just that element. We're often called "carbon-based life," but all-carbon molecules are either graphite or diamond. The carbon we contain is in molecules containing other elements: with hydrogen (and a pinch of oxygen), carbon forms fats. Sugars are made of carbon, oxygen, and hydrogen in special proportions. Workers are made from carbon, hydrogen, oxygen, nitrogen, and a tiny bit of sulfur also in special proportions, depending on their type. Readers interested in the details should look at a chemistry book, as the rules for how molecules are made could fill their own web site.

Atoms are so small it's difficult to understand their size. A sheet of paper is about one million atoms thick. Another way we can look at the sizes of extremely tiny things to compare them to things we can see. For example, the largest of human cells is about as big across as a human hair is wide. If that cell were to be considered to be a sports stadium, the typical worker would be as big as the ball, and atoms would be about the size of marbles:

Each cell-stadium wouldn't contain just a few worker-balls, but would be packed with them, all floating around in a sea of water molecules.