Mix 20 marshmallows with 2 tablespoons milk in top of double boiler; add 3 tablespoons lemon juice, cool slightly. Add 1 cup frozen raspberries (or use 1 cup thawed peach slices, chopped, or 1 cup sliced strawberries, thawed). Fold in 1 cup whipping cream, whipped. Freeze in refrigerator tray 2 hours. Spoon into 6 or 8 parfait glasses and top with berries.