The straw wrapper worm or caterpillar is a fun and easy science experiment. Your kids will love watching the “worm” grow right in front of their eyes.
My kids love learning about the butterfly life cycle and how the caterpillar transforms into a beautiful butterfly. We had a lot of fun using pasta to make each stage of the butterfly life cycle and used coffee filters to make colorful butterflies.
We also got a butterfly growing kit and watched the caterpillars eat and eat until they form a chrysalis. There is nothing like watching the metamorphosis take place in person and seeing the butterflies bust out of the chrysalises. I highly recommend doing this with your kids as part of your butterfly unit study.
When you think about it, the caterpillar actually does most of the work out of all 4 stages of the butterfly life cycle. They have to eat an enormous amount of food and can grow up to 100 times their size in preparation for the chrysalis phase.
This straw wrapper worm water trick is a great caterpillar activity for your kids to learn about the caterpillar as well as capillary action. When you add water to the straw wrapper, it will grow and move just like a worm!
I do want to note that caterpillars are not worms. That said, they have very similar movements and they are both cold-blooded!
How to Make a Straw Wrapper Worm
- Disposable plastic drinking straws that are individually paper wrapped
- Optional: permanent markers
- Optional: plate (we did the experiment on the floor but you can avoid getting water all over by doing it on a plate)
1. Crumple the straw wrapper and slide it off the straw.
The best way we found to do this is to take one end of the straw wrapper and scrunch it toward the middle of the straw. Then turn the straw around and repeat with the other side. You should end up with the straw wrapper all squashed in the center of the straw. Place your fingers behind one end of the wrapper and gently slide it off the straw.
You can also push the wrapper down the straw until it slides off the end. This way it’s a little harder but doable.
The tighter you scrunch the straw the more impressive the trick will be.
2. Draw on the straw wrapper with permanent markers. This is optional but it makes the wrapper look more like a caterpillar or worm. Plus kids love customizing their own wrappers! Make sure you use a permanent marker because you will be soaking the wrapper in water.
You can also decorate the straw wrapper before you slide it off the straw. This might be an easier approach if you are doing this experiment with younger kids who would have trouble keeping the wrapper scrunched while drawing on it.
3. Using a dropper, suck up some water and drip a couple of drops on the straw wrapper. What happens?
If you don’t have a dropper, you can also use the straw. Dip the straw into the water and seal the other end with your finger. This will hold the water in the straw while you position the straw over the wrapper. Then simply let go of your finger and water will come out of the straw.
4. Continue to drip water onto the straw wrapper until the wrapper stops expanding.
My kids were amazed when they saw the straw wrapper started to grow. The way the straw wrapper straightened out as it absorbed water looked a lot like the movement of a worm or caterpillar.
We ended up going through a whole bunch of straw wrappers because my kids just wanted to keep doing this science experiment. We saved all the straws for future experiments.
How Does the Straw Wrapper Worm Work
The science behind the straw wrapper worm has to do with one of our favorite physics phenomenons – capillary action.
The straw wrapper is made of paper. Paper is made from trees, which contain lots of plant fibers, and these fibers overlap to create a massive network of tiny tunnels.
Water molecules tend to want to stay together because of cohesion, and at the same time, they want to stick to another substance due to adhesion. Therefore, when you add water to the straw wrapper, water molecules start traveling through the microscopic fibers and spaces in the paper.
As the fiber tunnels fill up with water and expand, the paper straightens out. This causes the illusion that the straw wrapper worm is growing and moving.