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 COOL. CAN BE USED UP TO 50 TIMES. ALSO CAN BE USED AS A COLD PACK. COMPLIES WITH CPSIA, PROP 65. 3.1875" L X 3.75" W X 0.5" H