The Companions of Molecules
Common sense tells us that a molecule of water is not the same as a molecule of vinegar; the molecules of each are extremely small and in rapid motion, but they differ essentially, otherwise one substance would be like every other substance. What is it that makes a molecule of water differ from a molecule of vinegar, and each differ from all other molecules? Strange to say, a molecule is not a simple object, but is quite complex, being composed of one or more smaller particles, called atoms, and the number and kind of atoms in a molecule determine the type of the molecule, and the type of the molecule determines the substance. For example, a glass of water is composed of untold millions of molecules, and each molecule is a company of three still smaller particles, one of which is called the oxygen atom and two of which are alike in every particular and are called hydrogen atoms.