1/4 c. nonabrasive household cleanser (such as Bon Ami)
paper cup
water
2 or 3 drops food coloring

Pour cleanser into cup. Add water to make thick paste. Add food coloring and mix thoroughly (makes 1/4 cup). Give paint to child to play with in bath tub, shower walls, and side of tub. Paint washes off easily, taking bath tub ring with it.