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Amazing pencil trick

For kids: When a bag of water and a sharp pencil collide, the results may surprise you.

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Piercing plastic: A boy demonstrates the trick, showing how the water stays inside the bag, even though the pencils have made holes in the plastic.

Julie M. Prince

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What will happen if you poke a plastic bag of water with a sharp pencil? Try this experiment outside or over the kitchen sink to find out!

Materials Needed:

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1 zippered plastic storage bag
Several sharpened pencils


1. Fill the plastic storage bag with water until the bag is about three-quarters full and then seal it.

2. Hold the bag of water at the top with one hand. With the other hand, push a well-sharpened pencil through the bag so that the pencil is sticking out on both sides. See how many pencils you can add until the bag leaks.

What happened? Why didn't the bag leak when pierced with a pencil?

Plastic bags are made up of polymers, or chains of molecules, that bend and stretch. When you poked pencils through the bag, the polymer strands shifted aside to allow the pencils through. They quickly re-formed around the pencils, keeping water sealed inside.

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