Set up six glasses, fill alternating ones with red, yellow, and blue water, connect them with paper towel strips, and then wait. Over the next hour, the water silently "walks" along the paper towels, rises into the empty glasses, and begins mixing — red and yellow meet to make orange, yellow and blue create green, blue and red produce purple. By the time you check back, all six glasses contain color and a full rainbow has been created purely by capillary action. It's one of the most beautiful and educational science experiments for young children.

What You'll Need

• 6 clear glasses or cups arranged in a row or circle
• Water
• Red, yellow, and blue food coloring
• Paper towels (full sheets, not half-sheets)
• Patience (the best results come in 1–2 hours)

Step-by-Step Instructions

1. Arrange 6 glasses in a row: Glass 1, Glass 2, Glass 3, Glass 4, Glass 5, Glass 6. (Or arrange in a circle for a continuous rainbow loop.)
2. Fill glasses 1, 3, and 5 halfway with water. Leave glasses 2, 4, and 6 empty.
3. Add food coloring: 10 drops of red to glass 1, 10 drops of yellow to glass 3, 10 drops of blue to glass 5. Stir each.
4. Fold paper towels into long thin strips by folding lengthwise several times.
5. Connect the glasses: Place one end of each paper towel strip in a colored glass and the other end in the adjacent empty glass. Each empty glass should receive strips from both neighboring glasses.
6. Watch and wait. The paper towel begins absorbing colored water immediately, but visible mixing takes 30–60 minutes. Check back every 15 minutes and observe the progress.

The Science: Capillary Action

Water molecules stick together (cohesion) and stick to other surfaces (adhesion). In thin paper towel fibers, adhesion is stronger than gravity — water molecules pull each other along the fibers and climb upward into the empty glass. This is capillary action: the same phenomenon that draws water up through plant roots and stems, and through fabric wicks in candles. The rainbow forms because the three primary colors (red, yellow, blue) mix to form secondary colors (orange, green, purple) in the connecting glasses.

Questions to Explore

• How long does it take the water to reach the top of the paper towel?
• What happens if you use hot water vs. cold water?
• What if you use a thicker paper towel?
• Can you predict what color will appear in the empty glasses before it happens?

Frequently Asked Questions

How long does the walking rainbow experiment take?

The initial water movement is visible within 5–10 minutes. Significant color transfer into the empty glasses takes 30–60 minutes. The full rainbow effect, with clearly mixed secondary colors in all glasses, is most impressive at 2–4 hours. Set up the experiment at the start of a morning activity period and return periodically — the gradual change is engaging to check on throughout the day.

Why isn't the water walking in my experiment?

The most common issues: (1) The paper towel isn't touching the water at both ends — ensure each strip dips into the colored glass and reaches into the empty glass. (2) The paper towel is too thick — fold into a thinner strip. (3) The glasses are too far apart — the paper towel bridge shouldn't sag; position glasses so the strip curves gently rather than drooping. (4) Too little food coloring — use at least 10 drops per glass for visible results.

Can you use the walking rainbow experiment to teach color mixing?

Yes — it's one of the best demonstrations of primary-to-secondary color mixing available. The mixing happens gradually and visibly, making it much more impactful than simply explaining that blue plus yellow makes green. Have children predict what color will form in each empty glass before the experiment, then observe whether their prediction was correct. This predictive reasoning is authentic scientific thinking.

Related science: Color Mixing with Water | Baking Soda Volcano | Coffee Filter Butterflies (color science)