Physics at Mealtime: Capillary Action

This is a fun activity to try when you're waiting for the waiter to bring you your food, but consider yourself warned that not all people think it's appropriate to play with your straw at the dinner table…

What you Need

  • Drinking straw in a paper wrapper (like the ones you get at a fast food restaurant)
  • Water

What to Do

  1. Tear off one end of the paper wrapper. Push the paper wrapper down all the way to the other end of the straw so that the wrapper is all scrunched up.
  2. Slide the scrunched up wrapper off the straw and put it on the table.
  3. Use your straw to put a couple of drops of water in the middle of the wrapper. What happens?

dripping straw

What’s Going On?

puddleThe paper wrapper absorbs water that it comes in contact with, like a sponge. Did you notice that the water traveled along the wrapper as it wiggled and stretched out? Even though you only added water to the middle of the wrapper, most of the wrapper was probably wet by the time the wrapper stopped moving.

puddleAs we've discussed in other activities, water molecules have a positive end and a negative end; this means they are attracted to opposite charges on other molecules. Water molecules can be attracted to each other (which is why they stay in puddles) and to molecules of other substances, like paper (which is why they spread out when you put a paper towel over a puddle).

When you put a paper towel over a puddle, the water is attracted to the paper towel and spreads out.

When you drop it on the wrapper, a water molecule will try to get as close as it can to the surface of the paper. And since water molecules are also attracted to one another, it will drag its friends along. This is called capillary action and is why the water travels along the wrapper. The wrapper wiggles and stretches out as the water moves because as its fibers absorb water they swell and straighten out.


Try This!

  • Instead of using water to make the wrapper grow, try other kinds of liquid. Which liquids work the best?
  • What other things in your house can liquids climb by capillary action? (Hint, what else soaks up water?)
  • Can you make water travel up and out of a glass, despite gravity?

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