Fill a glass to overflowing with water and lay a postcard on it. Support the card with one hand, turn the glass upside down and remove your hand from the card. it remains on the glass, and allows no water to escape.
With a glass of normal height, a weight of water of about 2 ounces presses on each square inch of card. On the other hand the pressure of air from below is about one- hundred times as great on each square inch, and presses the card so firmly against the glass that no air can enter at the side and so no water can flow out!
Hanging water
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