Here's a fun and simple experiment for kids! A leak proof bag with pencils stuck in it?!

The Amazing Leakproof Bag – A Fun Experiment


This is the fourth week in a row of my husband, Jared, sharing a STEM related post on here! I know my kids are excited to have him doing all sorts of experiments with them and I’m glad he is filling in some of the hole in our science that are not as high on my priority list.

Today’s experiment is super simple and fun for the kids to do!



What happens when you fill a plastic bag full of water and then poke holes in it?  Not what you might expect actually! This is a super easy experiment that even young kids can do, is incredibly easy to set up and uses only materials you probably have around the house.

Before starting this experiment have everyone make a guess about what they think will happen when you poke a pencil into a bag full of water. Small experiments like this are a great way for kids to learn about developing a hypothesis (though you don’t have to use that word, especially with younger kids). Having them think about what might happen before experiments begin gets their minds working and helps them develop their logic skills.




Here's a fun and simple experiment for kids! A leak proof bag with pencils stuck in it?!



Have I mentioned that this is a super simple experiment to do? All you need for this is 3 things:

  • resealable plastic sandwich bags
  • sharpened pencils
  • water

Yup, that’s it, three really easy to find things.



You may want to try this experiment outside or over a sink just in case things don’t go according to plan.

  1. Fill a resealable plastic bag about 1/2 to 3/4 full of water.
  2. Hold the top of the bag with one hand, and with the other hand push a pencil into the bag. The pencil should poke through both sides of the bag. Make sure not to push the pencil all the way through.
  3. Continue adding as many pencils to the bag as you want.


Testing the leakproof bag


If you want to up the stakes ask for a volunteer who would be willing to stand under the bag as you poke pencils into it! You may also find it interesting to experiment with different types of plastic bags and different pencils (rounded and ones with straight edges for example).


Testing the leakproof bag


What’s Happening?

Plastic bags like the one you probably used are made of a polymer. This polymer (called low-density polyethylene) is a long chain of molecules. An easier way to think of it would be to imagine each of these chains as a strand of cooked spaghetti. The bag is made up of a whole bunch of theses strands right next to each other. As you push a pencil in it pushes the strands apart and the pencil goes between them. The strands are flexible so instead of breaking they are pushed aside and create a temporary seal against the pencil. This causes the water to stay in the bag.

Of course, if you poke a hole in the bag without pushing the pencil into the hole it won’t seal and you will have a leak.


We have a leak!


Once they got the hang of it our kids were able to add a lot of pencils into the bag without having it leak. We tried with mostly straight sided pencils and they worked well, though the rounded pencils probably sealed a little better.


Testing the leakproof bag


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