Do you have a set of pencils and a food storage bag? Yes? Then you are all set for this no-prep science activity that is a great introduction to molecules.

No matter how many times we do this project with my kids, they find it entertaining. What kid doesn’t enjoy filling a bag with water and jabbing it with sharp pencils? If you explain the science behind it (scroll down for my easy approach), not only will it be a fun activity, but it also will count as a science class for the day.

While this activity is great for keeping kids entertained outside, it also can be done inside. Once you get the hang of it, you can even do it at your kitchen table.

One potential problem is when little kids poke the bag with a pencil and then change their minds and pull it back, looking for a better spot. Mini water fountain over the dinner table? Don’t stress! Just grab another pencil off the table and stick it through their hole. It will stop the waterworks.

Another problem arises when kids manage to stick a second pencil into the same area as a previous pencil. The bag can’t seal around the resulting double hole.

Therefore, I recommend that the first time you do this activity inside, do it over the kitchen sink or in the bathtub. But after some experience, you can do it anywhere. Ready?

No prep science: a bag of pencils

What you need

• A food storage bag (this is our favorite)
• A set of sharpened pencils (Pentels are our favorites)
• Water

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What to do

Fill your bag with water (about ⅔ full) and close it securely.

Hold the bag in the air while your kids (one at a time) poke a pencil into one side of the bag and out the other side.

Add more pencils.

The best part? Removing the pencils and watching water pouring out! Need a shower?

I’m sure you will hear, “let’s do it again!”

What will happen

Before you begin the activity ask the kids to guess what will happen to the bag when you stub it with the pencil.

The first time we did this activity, all of my kids said the bag would leak. Now, of course, they already know what will happen, so it’s all about figuring out how it works. They like to squish the water around, turn the bag upside down, and even toss it in the air. Use freezer bags for this kind of experimentation.

Let me tell you from experience that little sandwich bags can’t take any amount of rough jostling. But don’t just tell your kids. Let them find out for themselves.

The Science Behind the Experiment:

Molecules, Atoms, and the Alphabet of Life

The best way I know to explain atoms and molecules to kids is by using the analogy of the alphabet.

The alphabet is a building block of language in the same way as atoms are building blocks of matter (matter is anything that occupies space and has mass). Every solid, liquid, gas, and plasma is composed of atoms. Our bodies have billions of atoms.

The letters of the alphabet can be arranged together into words in the same way as the atoms can be arranged together into molecules. Molecules are groups of two or more atoms bonded together.

Polymer molecules may contain thousands of atoms all bonded together in long repeated chains. Plastic bags are a type of polymer.

When you stick a pencil into a bag, it pushes the molecules aside. They try to stretch back to their original position and end up fitting tightly around the pencil.

That’s why the bag doesn’t leak!

Try this experiment with grill skewers, sticks (expect leaks), and toothpicks (they won’t go all the way across).

Do polymers confuse your kids?

Tell your kids to imagine that they are small molecules and ask them to move around the room (or backyard) freely.

Now hold hands with your kids (the more kids, the better). Start moving around holding hands. You are now a long chain polymer. Your movements are not as free, right?

Do you have another adult around? Ask him to push his body between you and the other person in the chain taking as much space as possible while you keep holding hands. Do you have to shift aside from your original position? Do your hands stretch from your chain buddy, but your body wants to move back where you stood before? When you are pushed away from your spot, does your body want to pull back into the person who is pushing you away?

If you don’t have another adult to help you, ask the kids to stand close together holding hands and you play the role of a pencil. Try to get between them taking as much space as you can. (Expect lots of giggles).

Did you know that polymer chains can get tangled, stick to other polymer chains, dissolve in liquids, and do all sorts of other exciting things? But we will leave that for another lesson.

Want to learn more about atoms, molecules, and elements?

What are atoms? (Rookie Read-About Science)

What’s the World Made Of? (Let’s Read and Find Out Science) 

Molecule Mayhem by Tom Adams

Molecules: The Elements and the Architecture of Everything by Theodore Gray (this book is gorgeous!) 

The Magic School Bus Slime and Polymer Lab

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