We humans like to think of reality as a nice, stable place, where various stuff stays in the same place unless we want it to go somewhere else. But dream on. In reality, if you look at water at the molecular level, it acts like a bunch of puppies crowding into a dog bed, with molecules bumping each other and jostling for position. When a lot of water vapor is in the air, molecules will get bumped up against a surface and stick to it, which is why condensation forms on the outside of a cold drink on a humid day.
Conversely, when the air is drier, water molecules in your cup of water can get bumped up into the air and stick to other molecules that are floating around. That process is called evaporation. If the air is dry enough, more molecules will jump from your cup into the air than will stick from the air into the water. Over time, the water will continue to lose molecules to the air, and eventually you'll end up with an empty cup [source: NEWTON].
The ability of molecules from a liquid to get pushed into the air and stick to it is called vapor pressure, because the jumping molecules exert a force, just as a gas or a solid that's pressing against something would. Different liquids have different vapor pressures. A liquid such as acetone -- nail polish remover -- has a very high vapor pressure, which means that it easily evaporates and goes into the air. Olive oil, in contrast, has a very low vapor pressure, so it's not likely to evaporate much at room temperature [source: NEWTON].