Have a baked pie shell ready.

3 c. frozen berries
1/3 c. water
1 c. sugar
4 tbsp. cornstarch
1/2 tsp. lemon juice
2 tbsp. butter

Bring to a boil, stirring constantly until very thick and bubbly. Remove from heat. Cool and pour into baked pie shell. Chill and serve with Cool Whip.