Money doesn’t grow on trees is said to warn someone to be careful how much money they spend, because there is only a limited amount:

For example:

I’d love to buy you that new video game, but money doesn’t grow on trees, so we’ll have to wait until next month. 

Stop throwing your allowance around so carelessly; remember, money doesn’t grow on trees. 

She asked for a new car for her birthday, but her parents reminded her that money doesn’t grow on trees. 

You’re always asking for a loan, but money doesn’t grow on trees – you need to learn to manage your finances better.

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