No water is needed. Penetrating moist heat for deep, temporary pain relief. Features safety switch because we care. Arthritic and rheumatic pain is often temporarily relieved with the use of moist heat. The increases blood flow can relieve inflamed joints and maintain flexibility. Moist heat can temporarily relieve back pain caused by over-exertion, stress and strain. Moist heat relaxes muscle spasms. No Water is needed: This pack produces its own moisture using humidity in the air. There is nothing to boil or microwave. When to Use Your Thermophore Automatic Moist Heat Pack: Use your Thermophore pack at the first sign of joint pain or muscle spasm. Use it for the pain of over-exertion from sports, gardening or household chores. Four Thermophore Sizes: Standard (14 inches by 27 inches) for an entire arm, leg or back. Medium (14 inches square) for a shoulder, elbow or knee. Petite (4 inches by 17 inches) for TMJ, neck and sinus. Muff (8 inches x 17 inches ) rolled, for stiff and painful hands. Hand crafted in the USA.