Slice enough rhubarb to cover 9 x 13 inch pan. Sprinkle 1/2 cup sugar over rhubarb. Sprinkle 6 ounce of any red Jello next. Pour 2 cups warm water over all. Sprinkle 1 package yellow cake mix (dry) next. Drizzle 1/2 cup melted butter over cake mix. Bake at 375 degrees for 45 minutes or until bubbly and brown. Cool and refrigerate.