Smooth, easy-to-make and able to set in nearly any shape, Jello-O is practically foolproof -- until you try to release it from its mold in the absence of a vacuum. When Jello-O sets in a typical gelatin mold -- those made of silicone and polymers -- it leaves no space for air; no air, no vacuum and nothing to break the seal holding the gelatin in. A little heat warms Jell-O just enough to release it from the mold without altering its shape.
Chill a plate or tray in the refrigerator for about 15 minutes. Press the Jell-O gently to check for firmness; it should spring back easily. If it doesn't, chill it in the refrigerator until it firms.
Fill a bowl or pan with enough warm water to reach to the top of the mold but not over it. Use a bowl for individual molds and a pan for tray-style molds.
Dip the exterior of the mold in the warm water for 10 seconds. Remove the mold form the water and shake it slightly.
Place the chilled plate on top of the mold. Invert the mold and the chilled plate -- one hand on top of the plate and the other under the mold -- and set them on the work surface. Gently pull the mold away from the gelatin.