From the Super Mario Wiki
In one of Koopa Koot's favors, he says that he has been dragging his shell recently and want a "tart pick-me-up", for which he thinks a Lime should do the trick. If Mario brings him a Lime, he will say "Mmmmm... Yow!
What a sour lime!
That'll put a spring in my step", and reward Mario with three Star Pieces.