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. WILL STAY WARM FOR 15-25 MINUTES. SIMPLY IMMERSE IN VERY HOT WATER UNTIL CONTENTS BECOME FLUID AND ALLOW TO COOL. CAN BE USED UP TO 300 TIMES. INSTRUCTIONS ARE PRINTED ON ITEMS. COMPLIES WITH FDA. 3 7/8" L X 3 3/16" W X 1/2" H