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Leak Proof Bag Experiment

Leak proof bag experiment for kids. Science experiment demonstrating polymers.

Science activities are the best, but it is extra fun when it seems to work like magic. And this STEM experiment is so simple, but huge on the magic factor. (Don’t worry, the explanation is simple too.)

This science activity just needs a resealable plastic bag, water, and pencils. You can do this at home or with any elementary grade at school.

Oh, and kids seem to really like having permission to jab something. That’s a big draw with this activity too.

Let’s do the leak proof bag experiment.

text: Leak Proof Bag Experiment Science Activity for Kids Picture: plastic resealable bag filled with water with 3 pencils jabbed through it, no water leaking

Leak Proof Bag Experiment

This experiment is perfect for busy caregivers. It requires minimal effort but really lets children explore and utilize the scientific method in a very natural way.

Plus telling your kids you’re going to teach them how to put a pencil through a bag of water without spilling will really grab their attention.

(And if you like this experiment, be sure to check out the Exploding Baggie Experiment! A safe chemical reaction using kitchen supplies and the same type of bags.)

Supplies for Leak Proof Bag Science Experiments

Two things and some water, that’s all you need.

  • Sealable Baggie
  • Sharpened Pencils
  • Water

Yep, this is all you need! You can make this work with just one pencil, but having more make this experiment even more exciting.

How To Do The Leak Proof Bag Experiment

This science experiment is deceptively simple. It really only takes about a minute to do, but your child will be fascinated by it for much longer.

Step 1: Fill your resealable baggie with water. You want it to be pretty full, but you can leave a little headspace at the top. Seal up your bag.

Step 2: Stab the baggie with a pencil and leave the pencil in place. Yes, I am serious. No, it won’t leak.

You can add more pencils. When you are ready, go ahead and pull one or more out to see the water leak out of the holes!

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We do have to address safety with this one, as this experiment involves stabbing. This activity requires adult supervision.

Make sure your child knows to only jab the bag and nothing else. Keep the pointy end of the pencil away from anyone’s face.

It might sound counterintuitive, but this activity is actually safer if the pencil you use is sharp. A dull pencil will have a harder time piercing the bag and make it more likely that an injury could occur.


As I wrote in the safety section, be sure to use sharp pencils to make this activity work best.

You can use pens and mechanical pencils as well, along with colored pencils.

Use the gallon size bags if you have them to have plenty of room to add lots of pencils. This really ups the wow factor.

Do this activity over a sink or a large container. If you wiggle the pencils around or pull them in and out a lot there can be some slight leaking. And once you take the pencils out of the bag the water will leak out. Having extra containment or using the sink will limit potential messes.

Clean Up

Clean up for this activity is pretty quick and easy. Pull all the pencils out of the bag. (Hold it over another container or sink for this!) Let the water drain out then throw the bag away.

How The Leak Proof Bag Experiment Works

Let’s dive into the science of this activity. I know it kind of looks like magic, but it is actually a very simple explanation.


The science of this experiment is polymers. The baggie we use is made up of polymers. If we remember from slime making, polymers are long chains of repeating molecules, called monomers.

When we stab the bag with our pencil, the pencil tip works its way between those polymers to make a hole. Once the pencil is in place the polymers surround the pencil. This causes the pencil to act like a cork and no water is able to escape.

Fun fact: One of the most common polymer chains used in plastic bags is high density polyethylene.

Vocabulary To Know

There are a few key terms that are helpful to mention to your kids.

  • Polymer– large molecules made of up chains of monomers
  • Monomers– small molecules that make up polymers
Text: Leak Proof Bag Experiment  Top Picture: plastic resealable bag and 3 pencils; Bottom Picture: Bag from above filled with water and 3 pencils piercing through the bag without the water leaking

Utilizing the Scientific Method

This activity is super fun to just experience, but it is also a great one to work through the scientific method with. Here are some tips to talk through it.

  • Ask a Question: Ask your child, what do they think will happen when they stab the pencil into the bag of water?
  • Observe: Look over the baggie and pencil. What does your child observe about them?
  • Hypothesis: Let your child tell you what they think will happen when they put the pencil through the bag of water.
  • Experiment: Time to stab the bag! Let them give the experiment a try and see what happens. Try more than once. What happens if they put the pencil in slowly compared to quickly?
  • Data Analysis: Talk about what happened during the experiment. Did the water leak out? Or did it stay in the bag? Did they notice any patterns?
  • Form a Conclusion: Ask your child to tell you the results of their experiment.
  • Next Steps: We don’t have to stop at the conclusion. Ask your child what they would like to try next? Do they have any ideas to further the experiment?

Pencil in a Water Bag Experiment

This is one of those STEM activities that gets kids really excited about science. It’s simple but full of learning. And it is so much fun!

You have got to try this leak proof bag science experiment. Trust me.


Let’s find your next fun activity!

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text: Leak Proof Bag Experiment Science Activity for Kids Picture: plastic resealable bag filled with water with 3 pencils jabbed through it, no water leaking

Leak Proof Bag Experiment

Prep Time: 1 minute
Active Time: 2 minutes
Additional Time: 15 minutes
Total Time: 18 minutes
Difficulty: Easy

How to do the leak proof bag experiment! This simple science experiment wows kids and gives them an amazing chance to practice with the scientific method. Plus it is super fast and easy to do!


  • Resealable plastic baggie
  • Sharpened Pencils
  • Water


  1. Fill the baggie with water and seal.
  2. Take a sharpened pencil and push it through the bag. Leave it in place. The water won't come out!



This is a pretty safe activity, but as you are using sharpened pencils please use adult supervision. Remind your child not to aim the sharp point towards anyone's face or eyes.


Do this activity over a sink or a bowl of some sort. Once you remove the pencils the water will come streaming out of the bag.

Clean Up

To clean up remove the pencils from the bag and let the water drain out. Throw out the bag.

Text: Leak Proof Bag Science for Kids; Picture: resealable baggie of water with pencils pushed through the bag so the water doesn't leak

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