Teacher：We all know that heat causes an object to expand ;cold causes it to contract¹. Now，can anyone give me a good example?

John：Well ，in the summer the days are long，and in the winter the days are short.

老师：我们都知道热胀冷缩的道理。现在，谁给我举个例子？

约翰：嗯，在夏天天都长，在冬天天都短。