A greedy cup

How to make a Pythagorean cup at home

It’s a cup that spills your drink when you get too greedy!

Safe­ty pre­cau­tions



  • bot­tle;
  • straw;
  • test tube;
  • plas­ticine.

Step-by-step in­struc­tions

Cut the bot­tom off of a plas­tic bot­tle. Bore a hole in the cap and in­sert a straw ap­prox­i­mate­ly half­way. Put a test tube on the straw, mak­ing sure there is a small gap be­tween the base of the test tube and the top of the straw. Use plas­ticine to seal the straw and cap to­geth­er. Pour wa­ter into the re­sult­ing “cup” so that it doesn’t quite reach the top edge of the straw. So far, so good. But as soon as you add more wa­ter, it starts pour­ing out of the cup – and will con­tin­ue to drain un­til it’s all gone!

Process de­scrip­tion

The Pythagore­an cup (also known as the greedy cup) was in­vent­ed by Pythago­ras, an an­cient Greek math­e­ma­ti­cian and philoso­pher. Leg­end has it that it was orig­i­nal­ly in­tend­ed to help peo­ple drink in mod­er­a­tion, al­low­ing them to fill the cup to a cer­tain lev­el and no more. Fill­ing the cup high­er than this set point caus­es all the liq­uid to drain out of it. This cup seems quite or­di­nary, but it works ac­cord­ing to the prin­ci­ples of a siphon. The straw and test tube form a chan­nel, one end of which leads out the bot­tom, and the oth­er into the cup's in­te­ri­or. As soon as the liq­uid lev­el ris­es above the up­per edge of the straw, the siphon en­gages. As the wa­ter fil­ters out, the pres­sure in the test tube above the straw is slight­ly low­er than at­mo­spher­ic pres­sure, and thus at­mo­spher­ic pres­sure con­stant­ly push­es the flow to con­tin­ue un­til the cup is com­plete­ly emp­ty.