Boy are these neat! And lifesavers on a cold day. These tough vinyl bags are filled with a solution of sodium acetate. When the disc inside the pack is ‘clicked’, the liquid turns to a solid, crystalline mass that goes to 130°F instantly and, depending on conditions, will stay that way for a half-hour of heat where you need it. Neat technology, and easily recharged/reliquified by boiling. Can be reused up to 40 times. 3.25"×4.25".