Pythagoras Cup

Make your own Pythagoras cup and trick your friends with this disappearing water act!

  1. Ages: 9 - 16

  2. 30 minutes - 1 hour

  3. Messy

  4. Grownup needed


Materials you'll need

Step-by-step tutorial

  • Step 1

    Cut the end of the straw nearest the bend on a 45 degree angle. Then, cut the tip off of the pointy side.

    Photo reference of how to complete step 1

  • Tip

    Depending on the size of your plastic cups, you may also want to cut your straw shorter. Make sure you are cutting the end furthest from the bend if you are shortening your straw.

  • Step 2

    Bend the straw.

    Photo reference of how to complete step 2

  • Step 3

    Using the push pin, make a small hole in the center of the bottom of one of the plastic cups. Then, widen it using your scissors. The straw should just fit in the hole.

    Photo reference of how to complete step 3

  • Step 4

    Push the straw through the cup so that the bendy end of the straw is inside the cup. Then, hot glue the flat end of the bendy side of the straw to the bottom of the plastic cup.

    Photo reference of how to complete step 4

  • Step 5

    Turn the cup upside down and seal the space around the straw with hot glue.

    Photo reference of how to complete step 5

  • Step 6

    Place the clear plastic cup lid on top of the second cup.

    Photo reference of how to complete step 6

  • Step 7

    Then, carefully place the first plastic cup on top of the lid so that the straw goes through the hole in the lid.

    Photo reference of how to complete step 7

  • Step 8

    Now you are ready to experiment with your Pythagoras cup! Fill the third plastic cup with water and begin to pour it into the top cup. What happens if you fill the cup below the straw? What happens if you fill the cup above the straw?

    After you have made your first Pythagoras cup, try modifying it to make a trick cup to fool your friends! Instead of using a second cup to catch the water, cut the straw so that it is flush with the bottom of the first cup and ask your friends if they’d like a drink!

    Photo reference of how to complete step 8

  • Tip

    Add food coloring to the water to see the trick better!

  • Final result!

    What's going on?

    The Pythagoras cup is a fun example of a siphon, a device that uses gravity to drain a container of liquid. As the cup is filled with water, the short end of the straw starts to fill up, reaching the same height as the water in the cup. Once that water level reaches the bend at the top of the straw, some of the water begins to drain down the long end of the straw due to the pull of gravity. At this point, the straw begins to act like a siphon: the draining water in the long end of the straw creates an area of low pressure at the bend in the straw. The weight of the water in the cup can now push more water into the straw, and this water rushes towards the low-pressure area, up and over the bend in the straw and out of the bottom of the cup. Once this cycle has begun, the siphon will actually drain out all of the water in the cup.

    Legend has it that Pythagoras, the Greek mathematician who came up with the Pythagorean theorem for right triangles, invented the Pythagoras cup to encourage moderation when drinking wine. The Pythagoras cup will work like a regular cup as long as it isn't filled to its brim, but if a greedy person over-fills the cup, the siphon kicks into action and spills the entire contents of the cup onto the drinker. Today, it's best use is as a joke cup - see if you can trick your friends!

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