How do you confidently carry a full glass of water to someone without spilling? Hold the glass upside-down! Huh? The water will pour out! And you’d be correct, but for one small detail.
Before turning the glass over, cover it with a card or piece of paper. We must be joking, right? Let’s give it a try!
Pour water into the glass and cover it with a card or flat piece of paper.
Lightly holding the card in place (without pressing it in), quickly turn the glass upside-down.
And now let go of the card. The card doesn’t fall off, and the water doesn’t flow out. As long as the card remains in a horizontal position, you can carry the glass wherever you want without spilling. So what keeps the card from falling when it has the water sitting on top of it? Air pressure. The force of air pressure acting on the card from below is greater than the weight of the water. But what about the water inside the glass, you ask? Air displaced by the water remains trapped in the glass and–because its volume increases slightly–has a lower air pressure than outside the glass. Now you can amaze your friends by bringing them the glass of water they asked for–but upside-down!