Science Puzzle
Why Doesn’t the Water Fall Out?
Fill a glass right to the brim with water and lay a postcard flat across the top. Hold the card in place, turn the whole thing upside down over the sink, then take your hand away.
The card stays put and the water stays in the glass. Gravity is pulling down on all that water. Why doesn't it simply win?
The Answer
Because the air is pushing back harder. We live at the bottom of an ocean of air tens of kilometres deep, and all that air presses on everything, from every direction, including straight upwards on the underside of the card.
That upward push is enormous compared with the weight of a glass of water. Atmospheric pressure is strong enough to support a column of water around ten metres tall; your glass holds perhaps ten centimetres. The card's job is only to keep a seal, so no air can sneak up inside and break the standoff. The moment the seal fails, gravity gets its way and you get wet feet.
The principle: Atmospheric pressure acts in every direction, not just downwards. The upward push of the air on the card far exceeds the downward weight of the water above it.