Put rhubarb in greased 13x9x2 inch pan. Mix sugar and jello. Sprinkle over rhubarb. Sprinkle cake mix over that and sprinkle water over all. Slice butter on top of cake mix.
Bake at 350 degrees for 45 minutes. Serve warm with vanilla ice cream or whipped cream. Serve cold if you're lucky enough to have any left.