Prepare and bake the cake in 13 x 9 inch pan as directed on package. Remove from oven. Let cool in pan 15 minutes. Pierce cake with large fork at 1/2 inch intervals.

Meanwhile, stir boiling water into gelatin in small bowl at least 2 minutes until completely dissolved. Stir in cold water; carefully pour over pierced cake. Refrigerate 3 hours.

Frost with whipped topping. Refrigerate at least 1 hour until ready to serve. Decorate as desired with colored sprinkles or sliced strawberries if you used strawberry gelatin.