Pythagoras Cup

Materials: ★☆☆ Easy to get from supermarket or hardware store
Difficulty: ★★☆ Can be done by science teachers
Safety: ★☆☆ Minimal safety procedures required

Categories: Fluids and Surface Tension, Pressure and Fluids

Alternative titles: Tantalus Cup, Greedy Cup

Summary

The Pythagoras cup looks like a normal drinking cup, but if filled past a certain level, it uses a siphon to empty itself completely. This ancient invention shows the principles of siphoning and was originally designed to teach moderation.

Procedure

Pythagoras cups are commercially available. The following shows how to build one with everyday materials.

Cut the end of a bendy straw at a 45-degree angle, then trim off the sharp tip.
Bend the straw and insert the short end through a hole in the bottom of a plastic cup, with the bendy part inside.
Hot glue the straw in place inside the cup to prevent leaks.
Seal around the straw on the outside bottom of the cup with more glue.
Place this cup on top of a second cup or a lid so water can drain out of the straw.
Slowly fill the top cup with water.
Observe that the cup behaves normally if filled below the straw bend, but when filled above the straw bend, all the water drains out through the siphon.

Links

How To Make A Pythagoras Cup Kids Science - TheDadLab:

Pythagoras Cup (Greedy Cup) filled with Mercury - Periodic Videos:

📄 Pythagoras Cup - KiwiCo: https://www.kiwico.com/diy/stem/motion-mechanics/pythagoras-cup?srsltid=AfmBOoqtv6-4E9-9v0pEidQc66DsmkRhqrdLRDCY-B99f6bP3FXy0obi

📄 Pythagoras Cup - Tastes of History: https://www.tastesofhistory.co.uk/post/pythagoras-cup

Variations

Add food coloring to make the siphon effect easier to see.
Experiment with different straw lengths to change the “trigger level” at which the cup empties.

Safety Precautions

Use caution with hot glue guns—adult supervision required.
Avoid overfilling the cup on surfaces that could be damaged by spills.
Make sure the straw is glued securely to prevent leaks.
Do not drink from the trick cup if glue or non-food-safe materials are used.

Questions to Consider

Why does the siphon only begin draining once the water rises above the bend of the straw?
What force keeps water moving through the siphon even after the level drops below the bend?
How does this relate to modern siphons used in plumbing or fuel systems?
Why might Pythagoras have used this invention as a lesson in moderation?
What happens if the straw is too short or too long compared to the cup height?