Can water climb upward on its own? It can when paper towels are involved. Water molecules stick to paper fibers and pull more water along behind them. This process is called capillary action. You line up seven cups in a row. Fill every other cup with water dyed red, yellow, or blue. Then connect all the cups with folded paper towel strips. Over the next few hours, colored water creeps up and over into the empty cups. When two colors meet in an empty cup, they blend into a new color. By the end you have a full rainbow made entirely by water pulling itself from cup to cup.