"Setup two aquariums at the front of the class. One aquarium is empty and the other is half way full. Ask for two volunteers, give them equal sized cups and have them scoop out the water from their tank and put it into the other person's tank. Ask students when they are at equilibrium (not when equal amounts of water in tank but when the person with no water can scoop with cup). Change factors: increase the size of one person's scoop and see what happens."-Web site