Get three clear glasses. Fill two of them with water, about 2/3rd. Put in a few drops of yellow food coloring in one, and a few drops of blue in the other one. The two cups should have the same amount of water.
Set the two cups, with the empty one in between the two. Place a cloth strip cut from a t-shirt first in the blue cup, and the other end in the empty cup. One end of the second cloth goes in the yellow cup, and the other in the empty cup. The cloths should be just long enough to make a bridge between the cups, otherwise, the magic takes longer.
While you wait for the magic, get your little scientists to predict what might happen. What will happen to the cloth dipped in the water? What will happen to the empty cup? What will happen to the cups full of water?
In about 6-10 minutes see the results.
Discuss how water is drawn up through the plants roots like the cloth.