Why does an air bubble rise to the surface of a glass of water?

Bubbles are comprised of gases, which have a lesser density than water. Since they are less dense, they get pushed up to the surface, and they rise, lighter than the liquid around them. This is just like helium in air; helium is lighter than air, so it rises, pushed to the top by the pressure around it.

Air bubbles don't have as much density as water, so since they are less dense they get pushed to the surface of the water.