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Pencil in Water Bag Experiment

Learn about the science properties of plastic bags with this fun pencil in a water bag experiment. You only need a few household items to create this simple science experiment with kids!

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Water in a Bag Pencil Experiment

This simple science experiment explores the properties of water and plastic. What happens when you put a sharp pencil through a bag filled with water? It’s almost like magic that the bag doesn’t leak!

This is a great experiment to spark curiosity in kids of all ages.

Pencil in a Plastic Bag Experiment

What is the Science?

Here’s what you’ll observe: water does not spill from the holes in the bag. This is due to the properties of both water and plastic.

Plastic storage bags are made from polymers, which consist of long chains of molecules. The polymers in the plastic give it flexible properties, allowing it to bend and stretch. Learn more about polymers with our butter slime recipe.

When you poke a sharp pencil through the bag, it doesn’t simply create a hole, instead, it slides between the chains of molecules that make up the polymer. As the pencil pushes through, the plastic stretches and forms a tight seal around it. This seal prevents water from leaking out of the bag. This is only possible because of the elasticity of the plastic, which clings tightly to the pencil and maintains the seal around the pencil preventing water from leaking.

Follow-Up Experiments

You can extend this activity by asking kids the following questions:

  • Can you put more than one pencil in the bag at a time?
  • What happens if you use other materials?
  • Is the result still the same with an unsharpened pencil?
Pencil Through a Bag Experiment

Supplies Needed

  • Sharpened pencils – they must be sharp for this experiment to work
  • Plastic storage plastic bag – we used a 1-gallon size
  • Water
Pencils Through a Bag Experiment

Watch the Video Tutorial

How to Do the Pencil in a Water Bag Experiment

1. Pour water into a large plastic storage bag and seal.

Fill your plastic bag with water.

Pouring water into plastic bag

Seal the top of the bag.

Sealing plastic bag

Note – You may need help holding the bag up as you fill it because it can get heavy.

2. Gather your pencils.

You will need sharpened pencils.

Sharpened pencils

3. Poke a hole through the bag.

Take a sharpened pencil and poke it through the bag. Don’t hesitate or the bag may start to leak. 

You may find that it helps to have an adult hold the bag upright while the child pokes the pencil through.

Piercing pencil into bag

When pushing the pencils into the bag, be sure that both ends stick out from the bag. If you push a pencil in too far, it will leave a hole and leak.

Pencil pushed all the way through

Keep adding more pencils but not too many or it may start to leak. 

Pencils through bag

Once you’ve completed the experiment, try out the follow-up experiment prompts listed above!

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