7∶30-7∶35 �オ�

　　Why does pouring hot water over a tight jar lid loosen(松開)it?��

　　Hot water makes the metal of the jar lid expand(膨脹):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?fù)袈?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(變形)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.��

　　QUESTIONS��

　　1.Pouring hot water over a jar lid will ____.��

　　A.make it fit very tightly��

　　B.make it less tight��

　　C.not affect the lid itself in anyway��

　　D.make the jar expand��

　　2.This happens because the hot water ____.��

　　A.heats whatever is inside the jar��

　　B.causes the jar to grow smaller��

　　C.makes the lid slightly larger��

　　D.all of the above��

　　3.An object is made up of molecules that move ____.��

　　A.if the object becomes very hot��

　　B.whenever hot water is poured over it��

　　C.all the time��

　　D.only when the object is expanding��

　　4.Objects expand when they are hot because hot molecules ____.��

　　A.move faster B.make up more room��

　　C.move slowly D.both A and B��

　　5.From this selection we can tell that the molecules in a road ____. ��

　　A.are not much like those in a jar lid��

　　B.will not expand even in the hot sun��

　　C.are like the molecules in rails, lid, and other objects��

　　D.are like no other molecules��