It wasn't serious—it was all done in fun.

那不是认真的，全是闹着玩的。

You can learn English in fun.

你可以在趣味中学习英语。

I'm not saying it in fun.

我讲这话不是开玩笑的。

It was a pact made in fun long ago.

这是很久以前在开玩笑时定下的契约。