google了一下，品种还不少
发现里面有一种是以前女朋友送过我暖手的
可以重复使用，用完放锅里煮，就又能用了
Heating pads are made of a plastic pouch and a clear liquid. Inside the
pouch is a metal disk that crystallizes the liquid causing it to heat up.
Types of heating pads include electrical, chemical and hot water bottles.
Disposable chemical pads employ a one-time exothermic chemical reaction such
as catalyzed rusting of iron, or dissolving calcium chloride. The reagents
are kept in separate compartments within the pad. When the user squeezes the
pad, they break and the reagents mix, producing heat.
The most common reusable heat pads contain a supersaturated solution of
sodium acetate (CH3COONa) in water. Crystallization is triggered by flexing
a small flat disc of notched ferrous metal embedded in the liquid. Pressing
the disc releases very tiny adhered crystals of sodium acetate
