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Walking on Water

Acadia National Park

 

Did you know that some animals can actually walk or run across the surface of water? How do they do that? Here's the answer:

  • Big animals, such as geese and lizards, splash across the surface by dashing as fast as they can. Their big flat feet push down on—and break through—the water's surface. But the water beneath their feet pushes back, and this holds them up just long enough for them to take the next step. 
  • For smaller animals such as water striders, no dashing and splashing is necessary. The animals' weight makes tiny dents in the water surface, but their feet don't break through it. This allows them to walk around almost as if they were on solid ground, with little worry about sinking.

 

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But why doesn't the water surface break? It's all because of "molecule power"!

  • Molecules (MOHL-uh-cules) are the tiny, tiny bits of stuff that you, and almost everything else in the world, are made of. One thing molecules are good at is holding on to each other. Water molecules in a pond (or even in a water drop) hold onto each other equally.
  • But the molecules at the surface don't have any molecules above them to hold onto, so they hold onto each other extra tightly. And this creates what scientists call surface tension.
  • Picture a balloon filled with water. When you push in on the balloon with a finger, the rubber stretches but it doesn't break. The rubber in a balloon is acting like the surface tension on the surface of a pond. The water's surface tension isn't strong enough to hold up heavy animals, but it's plenty strong for the lightweights.

 

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To see water surface tension in action, try this scientific experiment!

  • Just fill a glass with water and gently place a straight pin or needle onto the surface.

Why it works:

  • The metal pin is heavier than water, so it would sink if you were to poke it through the surface.
  • But since you laid it down gently and didn't break through the surface tension, the tension is able to hold it up.
     

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