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.