Cold therapy works by constricting blood vessels, resulting in a decrease in blood flow - and promotes healing by reducing swelling. Heat therapy increases blood flow to the are to help relax muscles and decrease pain. Put them together and you've got a lot of natural healing at your disposal for relieving sprains, strains, tendinitis and other injuries, as well as stiffness and poor circulation. Core's Traditional Hot & Cold Comfort Packs are made from non-toxic biodegradable gel inside soft, "frost-free" covers. Just pop in the microwave or keep handy in the freezer, where they will stay soft and pliable.