Chop 4 rhubarb stalks. Cook with 1 1/4 cups sugar and 1/2 cup sliced strawberries in a saucepan over medium heat until tender, about 10 minutes; strain and return the cooking liquid to the pan. Stir in 3 cups cold water and the juice of 1/2 lemon. Pour into a 9-by-13-inch baking dish, cover and freeze until solid, about 4 hours. Scrape with a fork.