This is a sodium acetate reusable hand warmer. The flexing of the small metal disc initiates nucleation in the sodium acetate solution. The sodium acetate changes from liquid to solid. To “recharge” the hand warmer, you just boil it in water for 10-15 minutes.
Reusable Hand Warmers (Amazon Affiliate)
US: https://amzlink.to/az0CJFBlKwduk
UK: https://amzlink.to/az0xsHbnRm2WJ
CA: https://amzlink.to/az0YN1HxXEA2J
IT: https://amzlink.to/az04u21SqM4fo
DE: https://amzlink.to/az021K7IlrUcB
AU: https://amzlink.to/az0wND7YfoyLv
NL: https://amzn.to/2Z1d68r