A cup of water is made up of billions of tiny particles of water with spaces between them. These particles are called molecules.
The molecules in a cup of water are always moving. If the water is cooled, the molecules slow down and move less freely. If it is cooled enough, it changes to solid ice. But still the molecules are moving. If the water is heated, the molecules move faster and farther apart. The water takes up more room. If it is heated enough, the molecules move so far apart that the water becomes an invisible gas. When a little sugar is put in a cup of water, the sugar goes between the molecules of the water. It disappears. We say that it dissolves.
No one has ever seen a molecule of water. A water molecule is far, far too tiny to be seen with our eyes alone. It is far too tiny to be seen with even our best microscopes. But scientists are sure that water is made up of molecules from the way it acts. There is no other way of explaining how water can be either a solid or a liquid or a gas. There is no other way of explaining how such things as sugar can dissolve in water. And there is no other way of explaining how a cupful of water can get bigger or smaller.
Thousands and thousands of other substances are made up of molecules, too. But, of course, not all molecules are alike. Sugar is made up of sugar molecules. Oxygen is made up of oxygen molecules. Air is made up of oxygen molecules and of other kinds, too. And so it goes. Always the molecules are moving, and always there are spaces between them. Even in the solid wood of a table and the plaster on a wall the molecules are moving.
Most molecules are far too small to be seen. But a few kinds have been seen with powerful electron microscopes.
Molecules are made of even smaller particles called atoms. The atoms in a molecule may be all alike or they may not be. A molecule of water is made of two atoms of hydrogen and one of oxygen. A molecule of oxygen is made of two atoms of oxygen.
Today scientists are inventing many new materials. They are making some of them by making different kinds of atoms join in new ways to form new kinds of molecules.
