Walking water experiment

This walking water experiment is one of the best science activities you can do with kids at home. It works for preschoolers and older kids alike, and it only needs a handful of supplies you likely already have. Seven clear cups, paper towels, water, and red, yellow, and blue food coloring are all it takes. The result looks like a magic trick, but there is real science behind every drop.

I have done this experiment with many different age groups and it lands every time. Kids watch in genuine surprise as colored water climbs up a paper towel and drips into an empty cup. What makes it even better is that it teaches two things at once: capillary action and basic color theory.

What is the walking water experiment?

The walking water experiment is a simple science activity that shows children how water can move through a material without any pumps or pressure. You set up a row of cups, some filled with colored water and some empty, then connect them with strips of paper towel. Over time, the water travels from the full cups into the empty ones, mixing colors along the way.

The science for kids concept at the center of this experiment is called capillary action. This is what happens when water is attracted to a surface so strongly that it can move against gravity. The cellulose fibers inside a paper towel act like tiny tubes. Water molecules cling to those fibers through a force called adhesion. Because water molecules also stick to each other through a force called cohesion, they pull one another along for the ride. The water creeps upward through the paper towel, over the edge of the cup, and down into the next one.

Surface tension plays a role too. It keeps the water clinging to the paper towel fibers rather than falling off in the middle. Once the water reaches the top of the towel and starts heading downward, gravity takes over and guides it into the empty cup below.

The color mixing happens naturally as the water from two different cups meets in the middle. Red and blue make purple. Blue and yellow make green. Yellow and red make orange. By the end of the experiment, every cup that started empty will have its own color, and you have a rainbow in a row of glasses.

What you need

The supply list is short. You need seven clear cups or glasses, six strips of paper towel, water, and food coloring in red, yellow, and blue. More absorbent paper towels tend to speed up the process, but standard ones work fine. Cut or fold your paper towel strips so they are long enough to dip into one cup and reach down into the next, with a slight arch in between. Trim off any extra length so they fit cleanly.

How to set up the experiment

Line up all seven cups in a straight row. Fill cups one, three, five, and seven with the same amount of water. Leave cups two, four, and six completely empty. Add five drops of red food coloring to cups one and seven. Add five drops of blue food coloring to cup three. Add five drops of yellow food coloring to cup five.

Place a paper towel strip between each pair of neighboring cups. One end should dip into the water of the filled cup, and the other end should hang down into the empty cup beside it. Make sure both ends are actually touching the liquid or reaching into the cup where water will arrive. Repeat this for all six gaps between the seven cups.

Now wait. The first few minutes are exciting because you can watch the color begin to climb the paper towel fiber by fiber. But the full effect takes a few hours to develop. I usually set this up and then step away for a while. Coming back to a full rainbow of colors in every cup is the payoff.

Troubleshooting slow movement

If the water seems to be moving very slowly, check the water level in your filled cups. The water should be high enough that the paper towel strip is well submerged on that end. If the levels have dropped, top them up. You can also add a few more drops of food coloring to keep the colors from getting too diluted as they travel and mix.

Questions to ask while you wait

The waiting period is actually a great teaching opportunity. Ask your child what they think is pulling the water upward. Ask whether they have ever seen water move like this before. Wonder out loud together about what will happen in the empty cups.

Once colors start appearing, bring in some color theory. Red, blue, and yellow are primary colors. Ask your child what they think will happen when red water and blue water meet in the same cup. Let them guess before the answer shows up on its own. When the purple, green, and orange appear, talk about how those are called secondary colors and where they came from.

You can also ask your child to draw what they see happening. Some kids express what they understand better through drawing than through talking. A quick sketch of the cups and the traveling water can reveal a lot about what they noticed and understood. This pairs well with science activities for preschoolers that focus on observation and recording.

For kids who are ready for more structure, you can introduce a simple scientific method format. Have them write down a prediction before starting, observe what happens during the experiment, and then compare the result to what they expected. This kind of practice builds real scientific thinking skills over time.

What happens at the end

After a few hours, all seven cups will hold colored water. The two red cups on the ends will still be red. The blue cup in the middle will still be blue. The yellow cup will still be yellow. But the four cups that started empty will now hold orange, purple, green, and a second shade of one of those colors depending on the exact mix.

One fun extension is to ask what happens when all the colors mix together. You can move the cups into a circle and connect them all with fresh paper towel strips to see. When all the primary and secondary colors combine, the result tends toward a muddy brown or near-black. It is a good visual lesson in why mixing all paint colors together does not give you a bright new color.

Why this experiment works so well

This experiment earns a permanent spot in my rotation because it is genuinely low effort and high impact. There is no prep beyond gathering a few cups and tearing some paper towels. Cleanup is easy. And the science it covers connects to real-world things kids encounter, like why a paper towel soaks up a spill or how plants pull water up from the soil.

It also scales well. Younger kids can watch, make guesses, and enjoy the color show. Older kids can take notes, make predictions, and think more carefully about the forces involved. If you are looking for more ideas at this level, the 50 simple science experiments for kids roundup has plenty of options that work the same way.

For a rainy afternoon when you need something that will hold attention for more than ten minutes, this is a solid choice. Set it up, check back in, and let the rainbow do the work.

More science experiments to try

If your child enjoyed this one, there are plenty of other hands-on experiments worth trying. The volcano experiment is a classic for a reason. The sink or float experiment is great for younger kids and sparks a lot of natural curiosity. The lava lamp experiment also touches on density and liquid behavior in a visually satisfying way. Any of these pair well with the walking water activity if you want to build a simple science afternoon at home.