* A tumbler of water * A piece of cardboard to cover it
Take the tumbler of water and immediately put the piece cardboard over it tightly, making sure that there are no air bubbles. Turn the tumbler quickly upside down preferably over a sink and take away your hand is holding the cardboard tight slowly. Just observe whether the water spills out or the cardboard falls off. Then slowly allow in some air by opening the cardboard slightly and notice what happens then.
The air pressure outside the tumbler holds the cardboard intact, not allowing the water to escape. When air is allowed in it takes the place of water that makes the cardboard fall and the water too.
The water does not fall out of the tumbler when the cardboard is held tight and tilted. However when air is allowed to enter the water, the water tries to push out making place for the air making the cardboard and water fall.