Rainbow Paper Towel Experiment: Walking Water Science for Kids

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We had fun this week doing a few different kinds of rainbow science. Have you ever tried the rainbow paper towel experiment? It’s an awesome way to explore capillary action, color mixing, and absorption through a hands-on STEM activity!

This is an update on a post I did more than 10 years ago. I love freshening up my older posts with better pictures, but I also enjoy re-doing favorite activities with my younger kids. Seeing their joy and wonder makes it feel brand new every time!

In this easy science experiment, we explored how paper towels absorb water and how colors blend to form a walking rainbow. The kids loved these simple experiments and learned all about primary and secondary colors while observing the science behind absorption.

We actually did two versions of this experiment—both simple, fun, and using items you already have at home!

Rainbow Paper Towel Experiment: Walking Water

The first experiment we did is called the rainbow walking water experiment. This classic color-mixing experiment is perfect for introducing young learners to scientific concepts like capillary action, color absorption, and liquid movement.

Materials Needed:

6 clear cups or jars
Paper towels
Food coloring (or liquid watercolors) in red, yellow, and blue
Water

We used 6 small plastic cups for ours.

Watch it in action:

How to Do the Rainbow Paper Towel Experiment

Step 1: Set Up the Cups

Fill three cups with water and leave the other three empty.
Add food coloring to the three cups of water, using the primary colors: red, yellow & blue.

Step 2: Arrange the Cups

Place the cups in a circle or a straight line.
Put an empty cup between each colored cup.
Make sure the cups are close together so the paper towels can connect them.

supplies rainbow walking water experiment

Step 3: Fold & Insert the Paper Towels

Fold paper towels into long strips (folding into thirds works well).
Adjust the size to fit inside your cups if necessary.
Insert one end of a paper towel into the colored water and the other end into an empty cup.

Step 4: Observe the Color Mixing

The water begins soaking up the paper towel and moves toward the empty cups. You can see the water climb slowly up!
As the colors mix, you will see secondary colors form:
- Yellow + Blue = Green
- Red + Blue = Purple
- Red + Yellow = Orange

It takes time! Let your kids watch and check in throughout the day.

Tip: Using smaller cups makes the process faster!

My kids thought this was so cool. It started conversations about absorbing and also primary & secondary colors which is always great!

The Science Behind This Rainbow Paper Towel Experiment

Why does this happen? The walking water science experiment works due to a scientific principle called capillary action.

What Is Capillary Action?

Capillary action is the ability of a liquid to move upward through small spaces, seemingly defying gravity!

Plants use capillary action to pull water from the soil up through their roots and stems to reach their leaves. In this experiment, the paper towel acts like a plant stem, pulling water upward and transferring it to the empty cup.

Adhesion: Water molecules stick to the fibers of the paper towel.
Cohesion: Water molecules pull each other along as they move upward.
Gravity vs. Capillary Forces: The small spaces in the paper towel allow the water to travel against gravity.

This experiment also works because of cellulose fibers, which make up the structure of the paper towel. These natural fibers, found in plants, create tiny channels that allow water to travel upward through capillary action. This simple experiment helps kids visualize how water moves through plants and introduces basic principles of physics and chemistry!