(Copyright Lanney Atchley) Inverted cup with water surface tension sealing water in the cup. Used with permission.
What Can Keep Water from Falling Out of an Inverted Cup?
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APRIL 08, 2010
April Holladay, HappyNews Columnist

Half-fill a cup with water. Lightly put a piece of flat, rigid plastic on top of the cup. Nothing (but gravity) holds the plastic to the cup. Enlist a trusting friend and move the half-full cup of water over her head. Turn the cup upside down. Why doesn't the water fall out and wet your friend? Magic?
Q: If you fill a coffee cup half full with room temperature water and place a flat smooth piece of plastic on top and invert it, the plastic will not fall off. A recent television show said that it works because the pressure in the cup is less than atmospheric pressure. I don't believe that. Is there some other explanation for this phenomenon?
Walter, Someplace, World
A: The television-show explanation at best elucidates only secondary issues.
"I would say that air pressure doesn't have a significant effect in this phenomenon. I believe it is solely due to surface tension," emails physicist Erik Ramberg of Fermilab.
The main reason the water doesn't fall out of the cup is that a film of water forms a seal around the rim of the cup and blocks or, at least, greatly restricts replacement air from entering the cup. Essentially no water can fall out of the cup because no air can come into the cup to replace the falling water.
Water surface tension — caused by electrical bonds holding molecules together — keeps the cup water from falling and your trusting friend's head dry.
Water molecules are slightly polarized. One end of a molecule is more positive than the other end. So, water molecules throughout the cup cling to their neighbors — the positive end of one molecule hanging on to the negative end of another.
The molecules "don't act like just a bunch of marbles in a bag," Ramberg says. Instead, more like "very, very weak magnets in a bag. They will tend to stick together and resist pulling apart."
Like molecules (the water molecules in the cup) tend to stick together. Moreover, unlike molecules (the water molecules and the molecules that form the walls of the cup, for example) tend also to stick together, but more strongly even than the cup water molecules. Likewise, the water molecules and the unlike red plastic-lid molecules stick together more strongly than the cup water molecules. So the tightly-held water molecules along the rim of the cup and along the walls of the cup form a barrier restricting or preventing the flow of air into the inverted cup. The tightly-held molecules form a seal.
The seal prevents air from flowing in, which prevents water from falling out.
The seal isn't perfect. A drop or two of water may ooze out. Perhaps a small bubble of air may sneak its way into the cup through a crack not filled by tightly-held water molecules. But the seal is good enough to prevent the half-cup of water from falling out of the cup.
Try it. It's an easy experiment. Be sure to:
• wet the lip of the cup
• wet the plastic lid before you place it on the top of the cup
• carefully turn the cup and lid upside down
• carefully remove your hand that's holding the lid to the cup.
If you'd like to verify surface tension is the main explanation of the phenomenon — repeat the experiment but squirt a bunch of detergent in the water. Don't hold the inverted cup over your friend's head because the detergent will reduce force of the water surface tension and the seal will fail. The water will fall out — whoosh!
More Exploring:
How do insects walk on water?, WonderQuest
Further Reading:
Surface tension by R. Nave, HyperPhysics, Georgia State University