Make and cool cake as directed on the box for a 9 x 13 pan. Pierce the cooled cake all over with a fork. In a small bowl, stir jello, boiling water until smooth; add cold water. Pour over cake. Run knife around the sides of the pan to loosen the cake. Refrigerate at least 2 hours. Frost with Cool Whip and decorate with the fresh raspberries.
Variation: can use orange Jello and mandarin oranges, strawberry Jello and fresh strawberries, lemon Jello with fresh raspberries.