Put 2 cups fresh or frozen raspberries into a strainer set over a large glass pitcher. It must be glass because you’ll be pouring 1/2 cup boiling water over the raspberries. Press berries with a spoon until all juice has been released. Discard pulp. Yes, discard it. It’s full of seeds and not any fun to eat. Do the same with 3 cups fresh or frozen cranberries over the same pitcher. Refrigerate pitcher.
When juice is chilled pour into a blender and with blender running add 1 cup of ice cubes one cube at a time. When mixture is slushy, pour into a pitcher or punch bowl. Slowly stir in one chilled can of sugar-free lemon-lime carbonated soda or water.