Summer STEM Challenge – Rainbow Walking Water

Water appears to defy gravity in this experiment — but it's not magic, it's capillary action. Water molecules are polar, meaning they have a positive and a negative side, which makes them naturally attracted to each other. This attraction is called cohesion, and it's what causes water molecules to stick together and follow one another like a line of ducklings trailing their mother.

But cohesion alone doesn't explain why the water climbs upward. That's where adhesion comes in. Paper towels and napkins are made of tiny fibers with microscopic gaps between them. Water molecules are attracted to those fibers and stick to them, pulling themselves into the narrow spaces and inching upward along the pathways. The smaller the gap, the stronger the capillary action — which is why a thin straw pulls water higher than a wide one, and why the fiber bridges in this experiment work so well. As the water travels from cup to cup, it carries the food coloring with it.

When two colors meet in an empty cup, they mix to create a brand new color — yellow and blue make green, red and yellow make orange, and red and blue make purple. This same process happens in nature every day: plants use capillary action to pull water up from their roots, through their stems, and all the way out to their leaves.