Turn the oven on to 250 degrees. Place Styrofoam cup, bottom side up, on an old cookie sheet and place it in the oven. Open a window (there may be some fumes) and carefully watch the Styrofoam cups melt. It will take about 2 minutes to get the cups to look like little hats, but you can experiment with more or less time if you like. Immediately remove the cookie sheet from the oven and use a butter knife to gently pry up the edges of the cup from the cookie sheet if necessary.
Let Cool (this should anly take a few minutes).