A molecule is the smallest piece of a substance that is still that substance. One molecule of water is still water. If you break that piece apart, you do not have water any more — you have smaller bits called atoms. Most molecules are two or more atoms holding on to each other; a very few substances are just one atom, but school science still talks about them in the same family of ideas. This is a classroom and lab word. In news and everyday talk people sometimes use it loosely, as if it only meant “a tiny bit.” Though actually, a spoon of sugar already holds an enormous number of molecules, so “tiny” in ordinary life is still huge at this scale. You will meet it in chemistry, medicine, and talk about air and plastic. It is not a kitchen word, and it is not the animal mole, and it is not the chemistry unit called a mole, even though those words sit beside it on the page.