THIS UNIQUE HAND WARMER IS REFERRED TO AS, CRYSTALLIZATION TYPE, BECAUSE IT IS ACTIVATED BY FLEXING A SMALL METAL DISK INSIDE THE HAND WARMER THAT WILL PRODUCT HEAT, STAYS WARM FOR 15-25 MINUTES. TO REUSE, SIMPLY IMMERSE IN VERY HOT WATER UNTIL CONTENTS BECOME FLUID, AND ALLOW TO COOL. CAN BE USED UP TO 50 TIMES. CAN ALSO BE USED AS A COLD PACK. COMPLIES WITH CPSIA, PROP 65. 0.5" H X 3.875" DIAMETER