Space and Physics
PUBLISHED
Picture yourself sitting down for a dinner party with Pythagoras. As well as dodging endless talk about triangles, and maybe how beans are tiny humans, you may have to be on the alert for pranks, according to an old legend that places him as the inventor of the mischievous Pythagorean Cup.
The rest of this article is behind a paywall. Please sign in or subscribe to access the full content.The Pythagorean Cup, or greedy cup, works a lot differently to regular cups. With any normal drinking vessel, you fill it to your desired level and then drink it. With a Pythagorean cup, sometimes dramatically called a "cup of justice", if you fill it too high you will soon see your beverage drain out of the bottom of the cup before you have a chance to drink it.
While the cup is often attributed to Pythagoras, like the theorem bearing his name, he likely did not invent it nor discover the mechanism that made it work. They're still pretty cool, and certainly date back centuries.
So how do they work? If you watched the above clip, you will probably guess that the suspicious bulge in the center has something to do with it, and you'd be right. Inside the stem of the cup is a small tube leading to a U-shaped bend, which reaches down into the glass.
As you fill up the glass, as long as the water level does not reach the top of the tube your drink will remain in the glass. But once it reaches the central tube, it will begin to fall down it due to gravity. As it does so, the liquid forms a seal at the top of the tube due to surface tension of the liquid, allowing for the liquid to be siphoned out of your glass, and making you look like a massive wallop in front of Pythagoras.
The mechanism may sound fairly useless, but is used in everything from urinals to the fabric softener tray in your washing machine.
There are ways you could get around this prank if you should need to, for instance by putting a dense liquid like mercury at the bottom. Since the pressure of the water above it is not enough to push the water/wine/WKD Blue up inside the tube, no siphoning can take place.
But it might not be worth the potential mercury poisoning in order to avoid a prank.
