Once he starts talking about football, you can't stop him — it's his pet subject.

一旦他开始谈起足球, 你就别想拦住他,这是他爱讲的话题.

《简明英汉词典》

If we can set him off on his pet subject, he will go on for hours.

如果我们能使他谈谈他最得意的话题, 他会滔滔不绝地谈几个小时.

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