Before You Start
Get everything you need
🧰 Materials
7 clear cups or jars (plus extras for speed race)
Paper towels (several sheets)
Napkin and coffee filter for comparison
Water
Red, Yellow, Blue Food Coloring
Color Mixing Map printable
Observation Stopwatch Checklist printable
Timer
⚠️ Safety First!
This activity is safe to do independently — just watch for spills!
Food coloring can stain surfaces and clothing — use old newspaper
Wash hands if food coloring gets on skin (it washes off easily)
Keep cups away from table edges so they don't tip over
What You'll Do
🌉
You're going to set up a bridge of 7 cups connected by paper towel strips, and watch something that seems impossible — colored water climbing UPHILL, all by itself, with nothing pushing it!
This happens because of capillary action — water molecules love to stick to paper fibers and to each other, so they actually pull themselves up and across the paper towel bridges, slowly filling the empty cups.
As the colors travel and mix, you'll watch brand new colors appear where there was only empty water before — and you can even race different paper materials to see which one is the fastest water climber!
Walking Water Rainbow Bridge
Follow along carefully
Set Up 7 Cups
🥤
Set up 7 cups in a row. Fill cups 1, 3, 5, 7 with water. Leave 2, 4, 6 empty.
Tip: Line them up neatly so the bridges will be easy to place!
2
Color the Water
🎨
Color cup 1 red, cup 3 yellow, cup 5 red, cup 7 blue.
Tip: A few drops of food coloring go a long way!
3
Fold the Bridges
🧻
Fold paper towel strips lengthwise into narrow bridges.
Tip: Fold 3–4 times so the strip is narrow and sturdy!
4
Place the Bridges
🌉
Place one bridge between each pair of cups — dipping into both cups.
Tip: Make sure both ends are dipped well into the water!
5
Check In Regularly
⏱️
Check every 10 minutes. Record on your Stopwatch Checklist. At 30 minutes — do the empty cups have color?
Tip: Patience pays off — the colors slowly climb the bridges!
6
Full Reveal
🌈
At 60 minutes — full reveal! Cup 2 should be orange, cup 4 yellow-orange, cup 6 purple.
Tip: Try racing paper towel vs napkin vs coffee filter to see which is fastest!
Try It Different Ways
🎨
Little Kids (5-6)
Set up the cups and colors with help, then check in every little while to watch the colors slowly climb and predict what new color will appear next.
🔬
Big Kids (7-8)
Set up the full 7-cup bridge independently, recording observations every 10 minutes on the Stopwatch Checklist for the full hour.
🧪
Challenge!
Race paper towel vs. napkin vs. coffee filter side by side. Time how long each takes to move color into an empty cup — which material wins?
What's Happening?
🧒 Kid Explanation
Water molecules are super clingy — they love to stick to things, especially the tiny fibers inside paper towels! That stickiness is strong enough to actually pull the water up and across the bridge, even though gravity is trying to pull it back down.
As the water travels across the bridge into the empty cups, it carries its color along with it. When two different colored bridges both reach the same empty cup, their colors mix together right before your eyes and make a brand new color!
🔬 Grown-Up Explanation
This activity demonstrates capillary action, driven by the combined effects of adhesion and cohesion. Water molecules are attracted to the cellulose fibers in paper towels (adhesion) and to each other (cohesion). This combination allows water to travel against gravity through the narrow spaces between paper fibers, much like it travels through the xylem tubes of a plant from roots to leaves. As water from two differently-colored cups meets in a shared empty cup, the colors blend through diffusion, producing secondary colors.
This activity supports NGSS K-LS1-1 (plants need water and use it to survive) and 2-PS1-1 (observable properties of materials, including absorption).