Rainbow Rivers: Unveiling the Science of Capillary Action with Colored Water Bridges

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I have been so excited to try out this experiment! I wanted to wait until my youngest was ready to talk about primary and secondary colors so both kids could enjoy the experiment together.

The Rainbow Rivers experiment teaches what the primary colors are, creates and reveals secondary colors, and begins the conversation about capillary action, which is the ability of a liquid to flow in a narrow space without help from other forces (like gravity). It’s how water travels from the roots to all parts of the plant!

Are you RED-dy to YELLOW-bout the coolness of this BLUE-iful experiment? (Okay, I know that was a stretch).

Let’s do it!

How to make the Rainbow Rivers science experiment

Supplies you will need

For this experiment, you will need the following:

Here’s a children’s book about primary colors and mixing them to make new colors that would be great alongside this experiment.

Supplies needed for the Rainbow Rivers colored water bridge experiment

Before you start

I chose to put a towel down when I did this experiment with my kids, just in case there was a spill (I let them pour the water themselves).


Here is how to do this experiment with your child:

Step 1: Fill 3 of the small cups with water and food coloring

Fill up three of the cups or small mason jars with water. You should fill it as close to full as you can (keeping in mind the paper towel will displace some when inserted into the cup).

Add a few drops of red to the first cup, yellow to the third, and blue to the final cup.

Stir each cup to mix the food coloring. Make sure you use a clean spoon or craft stick each time so you don’t mix the colors!

Adding water to the first, third, and fifth cups and leaving the second and fourth cups empty
Food coloring added to the cups with water in them

Step 2: Fold paper towels

We used one of the half sheets of paper towels for each piece, so four half sheets in total.

Fold the paper towel in half lengthwise, then once again, so you have folded it into fourths.

Then fold it in half, so your paper towel will sit nicely over two cups and not try to pop out.

Fold the paper towel in half lengthwise
Fold the paper towel in half lengthwise again
Fold the paper towel in half
Four folded paper towels for the five cups in this experiment

Step 3: Place paper towels in cups

Once you’ve folded the paper towels, you’re ready to insert the paper towels into the cups.

Situate your cups so you have one with water, one without, one with water, and so on. If you want to mimic the colors of the rainbow, place the red cup first, then an empty, then the yellow, another empty cup, and finally the blue cup. You could add another empty cup and red cup to the end to make purple too.

Place one side of a paper towel into the red with the other half in an empty cup.

Next, place a paper towel between the empty cup and the yellow cup.

Keep going until you have a paper towel joining each cup.

Placing one side of the folded paper towel in one cup and the other side in the next cup in sequence

Step 4: Wait and watch

There’s a hidden lesson inside this experiment: patience!

We will have to wait for the colored water to transfer, so you could either walk away and let it work, stay and watch it, or take a video and speed it up later so your child can watch it happen.

Whichever way you choose, it might take a few hours, depending on the size of the cups you use (and consequently, how much water there is to transfer). You’ll know it’s complete when you have equal water levels across the cups, but you certainly don’t have to wait that long to see the blending of colors.

Final product. See how the water levels are the same in each cup?
Final product. We have colored tablets and used them with this experiment.

The science behind the Rainbow Rivers experiment

This experiment teaches:

  • Primary versus secondary colors
  • Capillary action
  • Fine motor skills

How it works

The Rainbow Rivers colored water bridge experiment works by filling three glasses with water and the primary colors (red, blue, and yellow). Two glasses remain empty. A paper towel, using capillary action, draws the water from the cups with water and starts to fill the empty cups, where we place a paper towel from two different primary colors to blend into a secondary color (orange and green).

Primary versus secondary colors

This experiment opens the conversation by discussing the primary colors (red, blue, and yellow) and how those colors can be combined or blended into the secondary colors. In this case, we are combining red and yellow to make orange and blue and yellow to make green.

Capillary action

This experiment demonstrates capillary action, which is the ability of a liquid to flow in narrow spaces without the assistance of, or even against, external forces like gravity. The water moves through the paper towel due to this action.

Understanding capillary action can lead to an appreciation for how plants use this for nutrient transport.

Fine motor skills

Pouring water, adding food coloring, and placing the paper towels all require careful coordination of hand movements. This helps develop fine motor skills.

More science experiments that exercise creativity to try out with your child

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