A: Tom walked away with Mrs. Smith's lipsticks.
B: I've known that. I asked him the reason, and he said he did this just for fun.
A:That naughty boy.
exp. To be for the pleasure or enjoyment of something.（好玩儿）
e.g. I entered the competition just for fun — I never thought I’d win.
e.g. ——Why did you say it if you didn’t mean it?—— For the fun of it. I just wanted to see his reaction.
——如果你不是真心的，为什么要说出来? —— 好玩罢了。我只是想看看他的反应。
e.g. He doesn't just write for fun; writing is his bread and butter.
e.g. Although he had only entered the contest for fun, he won first prize.
e.g. I breathed as I watched him walk away。
e.g. The man wrote out a check and started to walk away.