冰袋一捏，为什么就变冰？

The cold pack has a soft inner bag. 冰袋里藏着一个软软的小内袋。 Inside it is a white powder called ammonium nitrate. 里面装着白色粉末，叫硝酸铵。 When you squeeze hard, the inner bag breaks. 用力一捏，内袋就破了。 The powder dissolves in the water around it. 粉末溶进周围的水里。 Dissolving pulls heat out of the air. 溶解时，它会猛吸周围的热量。 The pack becomes ice-cold in seconds. 冰袋几秒钟就变得冰凉。