In A Medium Bowl, Combine The Chili Powder, Cornstarch, Lemon Juice And Water. Mix Together Well. Place The Meat In A Resealable Plastic Bag With The Marinade And Turn To Coat Thoroughly. Marinate In The Refrigerator For 6 To 16 Hours. Place In A Slow Cooker And Cook On High Setting For About 4 Hours, Or Until The Meat Is Completely Cooked And Shreds Easily.