said to warn someone to be careful how much money they spend, because there is only a limited amount

(提醒别人花钱时要谨慎)钱可不是从树上长出来的，钱可不是从天上掉下来的

A: Mum, I'd like a new bike.

妈妈，我想要辆新自行车。

B: I'll have to think about it - money doesn't grow on trees, you know!

我得想想看──你知道，钱可不是从天上掉下来的!