Expand and contract 热胀冷缩

Teacher: We all know that heat causes an object to expand and cold causes it to contract. 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。