This unique hand warmer is referred to as "crystallization type", It is activated by flexing a small metal disk inside the hand warmer, Stays warm for 15-25 minute, Can be used up to 300 times, Simply immerse in very hot water until contents become fluid and allow to col. Instructions are printed on item. Complies with CPSIA, Prop 65. 3.1875" L x 3.75" W x 0.5" H. Price includes a 1 Colour Imprint. Imprint area is 1.5" W x 1.0" H on Front. Call for multi-colour or multi-location imprints or for rush delivery availability.