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. Muff (8 inches by 17 inches) rolled, for stiff and painful hands. Petite (4 inches by 17 inches) for TMJ, neck and sinus.