Add 1 cup boiling water to 1 package of Jell-O in a small stainless steel bowl; stir to dissolve. Pour half of Jell-O into a 9 X 13-inch pan and place on a level shelf in refrigerator to chill. Set the small metal bowl with the other half of the dissolved Jell-O into a larger bowl of ice. When the layer has set in the shallow pan, the gelatin over ice will be syrupy (it doesn't take long, since quantities are small). Add a heaping tablespoon of sour cream to the Jell-O in small bowl and beat until well mixed, using hand or electric beater. Pour this over the clear Jell-O in the shallow pan. Return to refrigerator; when set, repeat, using a contrasting color of Jell-O. Green (lime-flavor) is a good color for the first layer. When the last layer has set, frost with a mixture of the remaining sour cream and the whipped topping. (Blending the two toppings gives them an excellent spreading consistency.) Keep dessert chilled. Cut in squares to serve. Note: Each layer should be set only enough to hold the next layer. If set too hard, the layers will not bond well and may slip when served.