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Brain Surgery in a Bag Polymer Chain Experiment

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Halloween science activities are a fun way to introduce science to kids. This “brain surgery” brain in a bag experiment is a twist on the leak-proof bag science experiment.

So, you’ve probably seen the leak proof bag science experiment. It’s one of the easiest kids science projects there is.

All you do is stick a pencil in a bag full of water.

Kids are totally impressed by this, so, it’s always a favorite.

Halloween science activities are a fun way to introduce science. This "brain surgery" experiment is a twist on the leak proof bag science experiment.

But somehow, we’ve never done it before. I didn’t want to do the same basic thing that a million other people have done, so I thought of a fun twist.

In honor of Halloween coming up, I thought we’d give it a gross twist.

Don’t miss the complete list of Halloween science experiments!

We turned our plastic bag into a brain.

If you would also like to gross some kids out (truly, it’s super fun), here is how we did it:

What you need for the leak-proof bag science experiment

Halloween science activities are a fun way to introduce science. This "brain surgery" experiment is a twist on the leak proof bag science experiment.

You only need a few things for this science activity!

halloween stem challenge cards

Leakproof Brain Experiment Science

This science project works with the science of polymers. The flexible polymer chains of the plastic move out of the way of the pencil, but fill in around it.

This makes a tight seal that prevents the bag from leaking until the pencils are taken out.

Leak proof bag experiment lesson plan

For background, we watched a video about a real brain.

It was not for the faint of heart. You can look up your own brain surgery video if you’re brave enough!

After watching the video, we were ready to do our own brain surgery.

Halloween science activities are a fun way to introduce science. This "brain surgery" experiment is a twist on the leak proof bag science experiment.

I looked up a brain diagram online and did a rough sketch of a brain onto the plastic bag with a permanent marker.

Halloween science activities are a fun way to introduce science. This "brain surgery" experiment is a twist on the leak proof bag science experiment.

Next, we filled the plastic bag with water and added a few drops of red food coloring and sealed the bag tight.

Halloween science activities are a fun way to introduce science. This "brain surgery" experiment is a twist on the leak proof bag science experiment.

To conduct the experiment, simply poke the sharp end of your pencils into one end of the bag (through the center of the “brain”) and out the other side.

If your pencils are sharp enough, the plastic will form a tight seal around the pencils and no water will leak out.

Halloween science activities are a fun way to introduce science. This "brain surgery" experiment is a twist on the leak proof bag science experiment.

After the project is complete, pull the pencils out (I recommend doing this part outside) and watch the “blood” seep out of the brain.

The kids were pretty grossed out by this part. I thought it was funny how grossed out they were getting over red water.

If you want to take this activity further, discuss why the water didn’t leak until the pencils were removed.

Have the kids fill out their STEM lab worksheet to complete this activity (linked below)

stem challenge worksheet

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 halloween stem challenges 

Save this idea for later!