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What is surface tension?

The short answer

Surface tension is the stretchy, skin-like layer on top of water. Every bit of water pulls on the bits next to it, and at the surface there's no water above to pull on, so the top bits hold on extra tightly to each other. That makes a thin, springy layer strong enough to hold up light things like pepper flakes, a paper clip or even an insect.

Chloe and Leo, at the bowl

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The longer answer

Inside a glass of water, each tiny bit is pulled equally in every direction by the water around it. At the top, there is only air above, so the surface bits are pulled sideways and down, which packs them into a tight layer.

That layer is why water forms round drops, why a glass can be filled slightly over the brim without spilling, and why some bugs can stand on a pond.

Soap weakens surface tension. It gets in between the water bits and loosens their grip, which is why the pepper rushes away from a soapy finger and why a floating paper clip sinks when soap touches the water.

Three things to know

  • Some insects can walk on water. A water strider is so light, with legs spread so wide, that the water's stretchy surface holds it up.
  • Water drops are round because water holds itself together. Every bit of water pulls on its neighbors, tugging a drop into the tightest, roundest shape it can make.
  • A paper clip can float if you lay it down gently. Lower it slowly and flat and the stretchy surface holds it up, until soap breaks the skin.

Try it at home: Float a paper clip

  1. Fill a bowl with water and let it settle.
  2. Lay a small piece of tissue paper flat on the water and place a paper clip on top.
  3. Wait for the tissue to soak and sink, leaving the paper clip floating.
  4. Touch the water's edge with a soapy finger and watch what happens.

Try counting how many drops of water you can stack on a coin before they spill over the edge.

Other questions people ask

What is surface tension in simple words?

It is water's stretchy top layer. The water bits at the surface hold on to each other tightly, making a thin skin that can hold up very light things.

Why does soap break surface tension?

Soap slips in between the water bits and loosens their grip on each other, so the stretchy surface gets weaker where the soap touches.

Quick quiz

Watch the episode first, then tap an answer.

  1. Then Chloe touched the water with soap. What did the soap do?

  2. Chloe tried the soap a second time. Why did nothing move?

  3. One of these makes water's top let go. Which one is it?

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