Empty glass is full of air

This is an experiment about water. If you put something into water, the water needs someplace else to move, so the water might rise and spill.

Water doesn't go into the glass because air inside it was trapped and the air was trying its best to push it out.

If you hold an empty glass right side up, it's also full of air, but the air has someplace to go. If you pour in water, all the air will go out.