Walking water experiment: a 30-minute capillary action lab for K–2

1. Set 6 cups in a circle, touching. Fill every other cup about three-quarters full of water, so full and empty cups take turns.
2. Add a few drops of food coloring: red in one full cup, yellow in the next, blue in the last.
3. Fold each paper towel lengthwise into a strip about two fingers wide. Bend each strip into an upside-down U, with one end in a full cup and the other in the empty cup beside it.
4. Before anyone touches it, ask: "What do you think will happen to the empty cups?" Students show a thumb up for "water will move", thumb down for "nothing will happen".
5. Watch for 5 minutes. Within minutes, color creeps up the towels. Ask: "Is the water going up or down? Which way does water usually go?"
6. Explain in simple words: paper towels are full of tiny spaces between their fibers. Water likes to stick to the paper and to itself, so it pulls itself up through those tiny spaces. Scientists call this capillary action. Plants use the same idea to move water up thin tubes in their stems.
7. Check again after an hour or at the end of the day. The empty cups fill with orange, green and purple. Ask: "Which two colors made the green?"

For younger students: focus on observing and naming colors, and have students draw before and after in a folded paper "book". For 2nd grade: test one change, such as paper towel versus a napkin or a coffee filter, and count how many minutes until color reaches the top. For English learners: point and say up, down, mix, wet and full with each step. For students with sensory or motor needs: offer a job that doesn't involve wet towels, such as adding the drops or timing the class, and let students observe from seats.

Quick check: hold up a dry paper towel and ask a student to show with their hand which way the water will travel and why.

If you try it, tell us how long your color mix took and which paper worked fastest.