Why does pouring hot water over a tight jar lid loosen it?
这句中没想明白loosen it作何解，it指代什么？

原文是这样的:

Why does pouring hot water over a tight jar lid loosen(松开)it?
　　Hot water makes the metal of the jar lid expand(膨胀):this makes the cover loose.
　　Most objects expand when heated because they are made up of molecules(分子).Heat makes molecules move faster. The hotter the molecules become, the faster they move and the more space they need.
　　This explains why we hear a click as we travel by train, or a gentle thump(重击声)as we drive on a concrete(混凝土)highway. The clicks and thumps tell us that we are passing over expansion joints put in to allow room for the metal rails and the concrete to expand. These joints prevent the rails and the roads from transforming(变形)as they expand under the hot sun. Like the molecules of the metal jar lid, the molecules of these materials need more space when they are hot.

loosen:使松开，这里意译为膨胀．it指代前面的a tight jar lid ．