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Tantalus Cup

A Tantalus cup looks like an ordinary cup, but a bent straw hidden inside forms a siphon. Below the red line it holds water normally; fill it any higher and the siphon starts and empties the entire cup. It takes a plastic cup and two bendy straws and is a perfect lead-in to how siphons work.

Written from Bruce's original workshop handout. Bruce's final review of this plan is in progress.

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Tantalus Cup
DifficultyBeginner
Build time20 minutes
CostUnder $2
SafetyEveryday care
Good forTeachers, Families, Makers

What it shows

Fill it past the red line and the whole cup drains away through its own bottom.

Key ideas: siphons, air pressure, gravity, potential energy

Safety (Everyday care). An adult should drill the hole in the cup. Use the hot glue gun with a grown-up nearby. Test it over a sink or bucket.
Construction

Build it

  1. Gather the materials

    You need a plastic cup, two bendy straws, and a drill with a bit slightly smaller than the straw.

    Step 1

  2. Drill the cup

    Drill a hole in the bottom of the cup slightly smaller than the straw's diameter, so the straw will be a tight fit. It can go in the center or near the edge.

    Step 2

  3. Slit the short end

    On the first straw (the siphon straw), cut a slit about 3/4 inch long into the short end above the bend.

    Step 3

  4. Add an extension

    Cut a piece from the second straw and push it into the slit end of the siphon straw. This lengthens the short leg so it will reach almost to the bottom of the cup.

    Step 4

  5. Point the long end

    Cut the long end of the siphon straw at an angle so it is easier to start through the hole in the cup.

    Step 5

  6. Secure the joint

    Put a spot of hot glue on the joint between the straws, or wrap it with a rubber band. This is optional but keeps the straw from coming apart.

    Step 6

  7. Install the siphon

    From inside the cup, push the long end down through the hole until the bend sits below the cup's rim and the short leg nearly touches the bottom. If it leaks, seal around the straw with hot glue or try a smaller hole in a new cup.

    Step 7

  8. Paint the red line

    Mark a red line around the cup level with the top of the straw's bend, and add a sign: 'Do not fill above the red line!'

    Step 8

  9. Test it

    Over a sink, fill the cup below the line; it holds water. Then fill it past the line and watch it drain completely.

    Step 9
The physics

How it works

The bent straw is a siphon. As long as the water is below the top of the bend, it simply sits in the short leg at the same level as the water in the cup, and nothing happens.

Once the water rises above the bend, it flows over the top and down the long leg, filling the whole straw. Now the column of water in the long leg outside the cup is taller than the short leg inside. Gravity pulls that longer column down, and air pressure pushing on the surface of the water in the cup pushes more water up the short leg to replace it.

The flow continues as long as the outlet is lower than the water surface in the cup, so the cup drains until the water level drops below the end of the short leg and air gets in. Water flows from the higher level to the lower one, from higher to lower potential energy. In principle, air pressure can push water up a siphon as much as about 33 feet above the water surface.

Using it

  • Set the cup out with its sign and see who cannot resist adding a little more water.
  • Have students research the Greek myth of Tantalus and explain why the cup is named after him.
  • If the cup is only half full, how can you start the siphon without adding more water? (With the straw at the edge, tilting the cup works.)
  • Once the siphon is running, can you stop it? Will it start again on its own?
  • Use it to launch a full siphon lab: which way does water flow when both ends are in buckets, and what happens when the levels are equal?
Go further

Variations

  • Tantalus bucket: Bruce made a five-gallon bucket version with PVC pipe, with the drain pipe ending flush with the bucket bottom so it sits flat. Students washed glassware in it, then started the siphon to empty it neatly into the sink instead of splashing it.
  • Placing the straw near the side of the cup lets you start the siphon by tilting the cup when it is almost full.
If it doesn't work

Troubleshooting

If this happensWater leaks around the straw.Try thisSeal around the straw with hot glue, or drill a smaller hole in a fresh cup.
If this happensThe cup does not drain completely.Try thisLengthen the short leg so it reaches almost to the bottom of the cup.
If this happensIt drains before the water reaches the line.Try thisThe red line is too high; move it level with the top of the straw bend.
Free PDFDownload this planPrintable build plan with photos ExperimentSiphonsFamily version, worksheets and lesson plan

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© Bruce Yeany · Homemade Science with Bruce Yeany · youtube.com/@YeanyScience
Free for personal and classroom use. Please don't resell or republish. Build and use at your own risk; follow the safety notes and supervise children.