One way scientists talk about matter or substance-that is, the stuff in the world-is in terms of pure substances and mixtures. Pure substances are substances that contain only one kind of molecule. Water with nothing else in it is a pure substance. This is because it contains nothing but water molecules. On the other hand, suppose we put a teaspoon of sugar in a glass of water and let it dissolve. Now we have a glass full of mostly water molecules with some sugar molecules here and there between the water molecules. So our sugar-water isn't a pure substance. It consists of more than one kind of molecule; thus it is a mixture.