A molecule is the smallest piece of a substance that still acts like that substance. Think of it this way: if you kept cutting a drop of water in half, again and again, you'd eventually reach a single water molecule. Cut it one more time and you'd no longer have water at all, just separate atoms of hydrogen and oxygen.

Molecules are made when atoms bond together. A water molecule contains two hydrogen atoms and one oxygen atom stuck together (scientists write this as H₂O). A sugar molecule is much bigger, with carbon, hydrogen, and oxygen atoms arranged in a specific pattern. Some molecules, like those in plastics, can contain thousands of atoms linked in long chains.

Everything around you is made of molecules: the air you breathe, the food you eat, even your own body. The molecules in perfume drift through the air until they reach your nose. The molecules in rubber give a ball its bounce. Scientists who study molecules, called chemists or molecular biologists, have discovered how to create new molecules that don't exist in nature, like the molecules in synthetic fabrics or medicines.