Leak-Proof Bag

Tuesday, 27 March 2012 0 comments

Explore the magic of polymers with some simple household materials.

The Leak-Proof Bag

Who would have ever thought that a plastic bag, some water, and a few pencils would frighten the thunder out of Mom? Learn how to poke a hole in a plastic bag filled with water and reseal it like magic. The secret has to do with a better understanding of the chemistry of polymers.


  • 5 sharpened pencils with round edges
  • Zipper-lock plastic bag
  • Water
  • A few paper towels


A bit of advice... practice this over a sink before you present it at the dinner table.
  1. Start by sharpening the pencils (if they aren't sharp already).
  2. Fill the bag 1/2 full with water and then seal the bag closed. Pose this question to your dinner guests: “What would happen if I tried to push one of these pencils through the bag of water? Will the water leak out and make a giant mess?” Yes... unless you know the scientific secret.
  3. Here comes the scary part. Hold the pencil in one hand and the top of the bag in the other hand. Believe it or not, you can push the pencil right through one side of the bag and half way out the other side without spilling a drop. The bag magically seals itself around the pencil. Sound impossible? Try it… over the sink the first time!
  4. Continue to show your science “spear-it” (get it?) by jabbing the remaining pencils through the bag.
  5. When you are finished, hold the bag over the sink and remove the pencils. Throw away the bag and dry the pencils.
A few helpful hints… Make sure the tips of the pencils are sharpened to a point. Be careful not to push the pencils all the way through the bag or your “spear-it” experiment will turn into a big “clean-it-up” activity.

How does it work?

The plastic bag is made out of long chains of molecules called polymers. This gives the bag its stretchy properties. The sharpened pencils slip between the molecule strands without tearing the entire bag. Believe it or not, the long chains of molecules seal back around the pencil to prevent leaks. Now that’s the "Spear-It" of science!

From: stevespanglerscience.com


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