You know, I'm not exactly sure where it comes from. I know it grows on the Grand Line somewhere. But Luffy ate one that turned him into rubber, which is ...a lot more useful than it sounds.

[...and she's not sure whether to tell Katara about their downside or not. It's not the smartest sort of information to give out to anyone, really. So for now, she'll leave that part out, though Sanji might have other ideas.]