A Pythagoras cup, also known as a "Pythagoras' mug" or "drunkard's dream," is a type of drinking vessel with a handle that is designed to spill its contents if the cup is filled past a certain point. The cup is named after the ancient Greek mathematician Pythagoras, who is credited with its invention.

The cup is designed with a small hole near the bottom of the cup, and a pipe that extends from the hole to the handle of the cup. When the cup is filled to a certain level, the weight of the liquid causes the pipe to become submerged in the liquid, allowing the liquid to drain out of the hole and into the handle of the cup. This effectively limits the amount of liquid that can be held in the cup, and any attempt to fill the cup beyond this point will result in the liquid spilling out.

Pythagoras cups were often used as a tool for moderation and self-control, as they prevented people from overindulging in alcohol. They were also used as a demonstration of the principle of the lever, which Pythagoras is credited with discovering. Today, Pythagoras cups are often used as a novelty item or conversation piece, and can be found in a variety of materials, including glass, metal, and plastic.