Why are gases compressible while liquids are not?

Gases have a lot of empty space between their molecules, so when you squeeze them the molecules can move closer together, making the gas smaller. Liquids have molecules that are already almost touching each other, leaving very little room to squeeze more, so their volume hardly changes. Because the molecules in a liquid are already packed tight, applying pressure mainly forces the molecules to vibrate a little more instead of moving them significantly closer. This is why a gas can be compressed a lot, while a liquid hardly compresses. For example, if you press a balloon filled with air, it shrinks a lot, but a glass of water stays about the same size even when you squeeze it.