When Pam the great magician
Was eating lunch one day,
Some magic fans came over
And would not go away.
The people wanted magic,
But what was Pam to do?
She had her plastic lunch bag
But knew some science, too.

Please note:  All chemicals and experiments can entail an element of risk, and no experiments should be performed without proper adult supervision.

Kids, you can try what Pam did with her zip-closing plastic bag, a sharp pencil, water, and paper towels. Fill the bag almost full with water and zip it closed. Hold the bag over a sink. Slowly push the point of the pencil through one side of the bag, through the water. Push the pencil all the way through to the other side of the bag. The bag should not spring a leak. Why not? Plastic sandwich bags are flexible because of the long, stretchy molecules called polymers from which they are made. When a sharp pencil is poked through the bag, the polymer molecules slide away, and then flex enough to squeeze back around the pencil. This makes a tight, leak-proof seal. It also works for a balloon. Lightly oil a knitting needle, then poke it through the top of a blown-up balloon and out the bottom near the tied knot. Rubber tires on cars also work this way. A gummy layer on the inside of the tire seals around any nails or sharp objects that poke directly into the tire.

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Kathleen Carrado Gregar, PhD, Argonne National Labs 
[email protected]
February 1998

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Taken From: Apples, Bubbles, and Crystals: Your Science ABCs, by A. Bennett & J. Kessler, 1996, McGraw Hill, NY.