Our Lava Pads are double fabric covered microwaveable heating pads (outer layer is tan or black flannel) that are over 1"" thick and use a water-based non-toxic fluid that contains an anti-freezing agent to allow it to retain a state of coldness below 32F, These same thermal therapy wonders can also provide evenly distributed heat for several hours after being heated for only a few minutes in a microwave, This type of moist heat therapy is great for soothing back pain joint stiffness fibromyalgia & arthritis treatment and many different types of muscle soreness, SKU: VSTR026