If molecules are in constant motion at terrific speeds, and this is taking place in everything — even a piece of wood — why can’t we see things changing shape?

A molecule is the smallest bit of a substance that can exist and still keep the properties of the whole. For example a molecule of sugar (sucrose) is the smallest bit of sugar that would still have the taste, color, form, solubility, and other qualities of sugar. If you divided it any more, it would no longer be sugar.

Now molecules of different substances vary greatly. Some are only a few billionths of a millimeter long and some are thousands of times larger. The molecules of the gases of which air is composed are so tiny that in a single cubic centimeter of air there are about 30,675,000,000,000,000,000 molecules!

But even though there are so many molecules in a substance. there is a great deal of empty space between them. Since molecules are always in motion, they move in a perfect vacuum. There is no air between the molecules of air, only a vacuum, and there is no iron between molecules of iron, only a perfect vacuum.

What causes the molecules to dash about is heat. The higher the temperature, the more violent the motion. In a hot gas, this motion is very violent. In a liquid or solid, it is much slower. But there is motion even in a piece of ice!

If the molecules in a substance are continually bumping into each other and pushing each other aside in all directions, why can’t we see the effect Of this motion? Why can’t we see a piece of iron, for example, being changed by the motion? Why does it seem so solid? The reason is that in a solid or liquid the molecules are held in their places by the forces of attraction between the molecules. Otherwise, the substance would be torn apart. The electrical forces which bind the molecules are strong enough to make most solid materials as strong as they are.

But if we apply intense heat, the motion of the molecules becomes greater and the substance becomes a liquid. When still more heat is applied, the molecules overcome the electrical forces, fly apart, and are transformed into a gas!