The 10″ Round heat pack is our personal favorite. This is our longest lasting heat pack. It can last up to 4 hours when properly insulated. The Round works great for cramps and sore muscles on any part of the body. You can put it on your car seat, take it to the ball game for a stadium seat or snuggle up on the couch with it or place it in your bed at night for hours of soothing heat. You can also use the heat pack as a cold pack. Just place it in the freezer for up to one hour and it will stay soft and flexible.