Walking water: build a rainbow
Can you fill a cup without pouring into it? Build paper bridges and watch water carry colours to its neighbours. There is time to observe and keep a little science notebook.
What should happen
Coloured water gradually appears in the two empty cups. Red and yellow usually make orange; yellow and blue make green. Actual shades depend on the dyes.
Let’s do it step by step
4 stepsSet out the colours
Line up five cups. Fill cups one, three and five about two-thirds with water. Colour them red, yellow and blue respectively. Leave cups two and four empty.
Build four bridges
Fold towels into strips 2–3 cm wide. Connect every neighbouring pair. Both ends must reach the bottom; in full cups they must be under water.
Notice the changes
Watch the paper become wet. After ten minutes, note how far the wet patch reaches. Check again later; water need not arrive in the empty cups immediately.
Compare before and after
Leave it for several hours or overnight if needed. Compare water levels and colours. Some water stays in the paper.
Now we know why
Ages 5–6
Paper absorbs water. Water travels through tiny spaces between its fibres and reaches the neighbouring cup.
Ages 7–10
Water is attracted to paper fibres and wets them. Narrow gaps help it climb: this is capillary action. The wet bridge carries water to the next cup, where colours mix.
Ages 11–14
Wetting, surface tension and attraction between water molecules support capillary rise. Once the bridge is wet, gravity and different water levels help the transfer. The towel does not create water. Tall plants also depend on the pull caused by evaporation from leaves.
This is not a complete model of water transport in tall trees; capillary action is only part of the story.
If it turns out differently
The paper stays dry
Make sure the ends touch water and try a more absorbent towel.
Water has not arrived yet
Shorten the bridges, bring the cups closer and allow several hours.
Colours look weak or different
Dyes mix differently. Next time compare dye amounts, changing only one variable.
A question for the explorer
Where did the water in the empty cup come from? How could we check that it travelled through the paper?
What if…
Compare two separate three-cup setups using different towels. Keep cups, water levels and bridge lengths the same. Compare after an hour.
After the experiment
Put wet paper in the bin, empty the water and wash the tray. Do not drink the coloured water.
Continue on screen
Change conditions in the interactive model and explore the science.
Open the model ↗Sources and images
Text and illustrations prepared for Veda. AI-created illustrations show the intended setup; real results may vary.