Step-by-step tutorial
Step 1
Cut the paper towel into ~1 inch strips.
Step 2
Draw a line towards the bottom of the strip, about 1 inch from the end. Use different colors for each strip.
Step 3
Add some water to the cups.
Step 4
Carefully place the paper strip into the water. The end with the color line should be in the water, but only the tip of the paper towel should touch the water.
Final result!
Watch what happens as the color moves up the paper towel.