The Leakproof Bag Science Experiment

You know that sinking feeling in your chest when you watch a horror movie and the suspense is killing you? I will warn you that this leakproof bag experiment will probably give you that same feeling of fear and anxiety, but it’s all in the name of science and education for our kids. 

Leakproof Bag with Many Pencils

And don’t worry, if you do this experiment over a sink or a large container, you should be able to avoid water spilling all over the place. Can’t promise you that it won’t spill at all, but at least you don’t have a watery mess on your floor.

What am I talking about? Let’s explore polymer chemistry with a plastic bag, water, and pencils!

How to Do the Pencil Through Bag with Water Experiment


  • Zip bag (any size will work)
  • Water
  • Sharpened pencils


  1. Fill up the zip bag about ⅔ full with water and close it up tightly.
  2. Push the pencils through the bag – in one side and out the other. 
Leakproof Bag Pencil Poking

Yup, that’s it! So simple yet so amazing! 

Your kids will want to do this over and over again because it’s like a magic trick. I lost count how many times we did this experiment. How many pencils can you poke through the bag without it leaking?

Leakproof Bag Lots of Pencils

The leakproof bag experiment is so simple that even young kids can participate. My 4 years old at first poked a hole in the bag with the pencil and took it out before I could stop her, and the water flowed out. She learned after that to push the pencil all the way through and we had no problems after that.

My super energetic son did get too enthusiastic at one point and stabbed at the bag so hard with the pencil that a large hole formed in the bag. Good thing we had a container underneath because water escaped right away. So lesson learned – push the pencils through SLOWLY so you don’t rupture the bag.

You can also have your child carefully remove the pencils over a sink or container and watch what happens. Just be ready to dodge some water because chances are your kids will want to squeeze the bag and water will shoot out of the holes! If the weather is good, you may want to take this experiment outside and water the garden with the water.

Science Behind the Leakproof Bag Experiment

Plastic bags are made out of polymer chains, or long chains of molecules. When you push a pencil through the plastic, the polymers spread out and increase the gap between the molecules, but does not break the chains. 

The polymer chains are flexible and form a temporary seal against the edge of the pencil. Therefore, water cannot escape and leak out of the bag. When you take the pencils out, the holes remain because the molecules do not move to close the gap in the polymer chain. As a result, the water leaks out (hopefully not all over the place).

Science Extensions

  • Different plastic bags have different thicknesses. Try a grocery bag instead of a zip bag or even a thick garbage bag. Did you get the same results?
  • Try poking a hole through the bag with pencils with different sizes and shapes. Which type of pencil works the best, or are they all the same?

For more super simple and fun science experiments for kids, check out:

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