Anyone who has taken high-school math classes has heard of the Pythagorean theorem (even if you don’t remember exactly what it was). Most people also remember that it was named after Pythagoras, an ancient Greek mathematician. Not surprisingly, this brilliant man was also responsible for many other concepts and ideas.

One of the things that he is often credited for that he almost certainly didn’t invent, however, is the Pythagorean cup, which is also known as the ‘cup of justice’ or the ‘greedy cup’ or sometimes the ‘civilized glass’.

This is a specially designed cup where when you fill it up with the drink of your choice, it works completely normally as long as you only fill the cup to a certain point. If you take more than you are supposed to, however, your entire beverage will leak out the bottom, exposing you as being greedy (or a drunk, depending on the situation).

So, how does this weird and inconvenient glass work? Well, the science behind it can be found in many other scenarios that you run into in everyday life. For example, if you’ve ever used a urinal, the chances are it harnessed this method. Also, your washing machine likely uses it to access the detergent or liquid fabric softener.

The way it works is by having a bulge going up in the center of the glass, which has a small u-shaped bend in it. The opening of the tube is near the bottom of the glass, then goes up, bends, and then travels down to the base of the cup.

When a liquid is poured into the glass, the tube will fill up and as long as you don’t fill it past the top of the tube, it will slowly drain out as you take drinks. If, however, you fill the glass higher than the top of that bend, the liquid will start falling down to the bottom of the glass and onto the table.

Once this begins, the surface tension and gravity will start a syphon, which will keep pulling more and more liquid out of the glass until it is empty, leaving behind a big mess on the table and an embarrassed guest.

To see it in action, check out this brief but interesting video:

Needless to say, this glass is quite impractical and can create a big mess. For those who are looking for a science-based prank, however, it can also be a lot of fun.

So, even though there is no evidence that this ‘Pythagorean Cup’ was invented by Pythagoras himself, it is a great example of how surface tension and gravity can work to make a syphon.

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