Magical Plastic Bag Experiment

leak proof plastic bag experiment

Magical Plastic Bag Experiment | TinkerLab

Here’s a fun experiment that won’t take a lot of time, and it’s more than likely that you have all the “ingredients” around the house. I did this with my three and a half year old, and it would be relevant for preschoolers and elementary-age children.

The idea that we’re testing here is what will happen if we poke a sharp pencil through a plastic bag of water. Will the water leak through the holes? Will the water spill out? Or will the bag reseal around the pencils, keeping the water inside?

When my 3-year old daughter (N) and I tried this out, we worked with the question, “what will happen if we poke pencils into a bag full of water?” That seemed more age-appropriate and tangible for her.

Magical Plastic Bag Experiment | TinkerLab

Supplies

  • Zip-up Bag
  • Water
  • Sharpened Pencils

We filled a zip-up bag about half-way with water and sealed it up. I held the bag high over a sink and N poked the pencils straight through the bag, from one side to the other. This is where my fancy photography skills come into play, holding the bag with one hand and snapping a photo with the other. Are you impressed?

Make sure that the pencil doesn’t keep traveling through the bag or you’ll have water leaks.

Magical Plastic Bag Experiment | TinkerLab

Keep adding pencils until you’ve had enough. Before removing the pencils, take a moment to talk about what you see. When the pencil goes into the bag, the bag seems to magically seal itself around the pencil.

Magical Plastic Bag Experiment | TinkerLab

When you’re done, remove the pencils over a sink.

The Science Behind the Experiment

Plastic bags are made out of polymers, chains of molecules that are flexible and give the bag its stretchiness. When the sharp pencil pokes through the bag, the stretchy plastic hugs around the pencil, creating a watertight seal around the pencil…and the bag doesn’t leak.

More Polymer Fun

Now I’m really excited for us to try poking skewers through balloons without popping them (QuestaCon Science Squad) and make our own kazoos from toilet paper rolls and plastic bags (Kazoologist). Steve Spangler Science is also an amazing place to go for projects like this, and you’ll find endless polymer-related ideas over there.

You could also make your own polymer by mixing up a batch of fun and flubbery Gak (a mixture of water, white glue, and borax). We’ve done this multiple times, and my kids can’t get enough of it.

More Science Projects for Kids

If you enjoyed this project, you’ll love this article: Science Fair Project Ideas.

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