When you drill a hole in a tin can and try to fill it with water, the water drips out because of gravity and atmospheric pressure forcing the liquid through the hole. However, when you put your hand over the cup, you are changing the system. Instead of atmospheric pressure applying a force to the can, it is applying a force to your hand. The air inside the hand-covered can is at atmospheric pressure, but after a little water is lost, it is not. The lost water increases the volume in the container (assuming your hand is airtight) and it forms an area of less than atmospheric pressure. Eventually, water can't escape because the force from atmospheric pressure (the force from air molecules bouncing off the container) is equal to the force of gravity and the force from air pressure from inside the container.
There's one other factor involved too. There's surface tension (from the tendency of water to stick together) that keeps the water at the hole in the form of a gentle bulge. Otherwise it would break up and drops would fall off. Mike W.
(published on 03/15/07)