|Did you notice that the colored
water begins to spread upward and away from the corner
of the paper?
How does the water actually move up
through the paper towel? It happens due to a force
of attraction between the paper fibers and the water.
This force is called capillary action. The water is
pulled up into the open spaces between the fibers
in the paper, soaking the paper as it absorbs the
water. When the weight of the water in the paper is
equal to the upward attraction, then the water stops
Capillary action is also the force by
which plants draw up water through their roots, and
by which sap rises in trees in the spring.
Adapted from Is Science Magic?,
by Ovid K. Wong (Childrens Press, Chicago, 1989).