Use hot therapy when motion is limited by stiffness. Heat increases blood flow to the affected area and improves range of motion. Use cold therapy when motion is limited by pain. Cold reduces flow to the affected area and controls inflammation. Durable construction for long life. Store in freezer for immediate cold therapy. May be heated in hot water or microwaved in water for hot therapy. Thick and flexible premium gel remains conformable at all temperatures. Formulated with non-toxic and non-caustic ingredients for safe use by everyone. Medium and large sizes maintain therapeutic temperatures for up to 30 minutes; small size up to 20 minutes.