Yeah, this has long been a mystery to Star Wars fans.

How do they grow plums on a world totally covered by desert?

EDIT: Holy cow. A reader pointed us at this. We're truly amazed. Mostly that none of us had known about this already.

Transcript

Shmi: What do Jedi knights do?
GM: Ah, I'm glad you asked. They're guardians of Justice throughout the Galaxy. They use the Force to gain semi-mystical abilities.
GM: The Force comes from midi-chlorians—
Shmi: Midi-chlorians?
Qui-Gon: They're microscopic organisms that exist in all living cells.
GM: Oh don't be ridicul... No, you know what? I'm going to run with it this time. Yes, they're microscopic organisms in your cells.
Qui-Gon: They continually speak to us, telling us the will of the Force.
GM: Sure, why not?
Qui-Gon: When our minds are quiet, we hear them speaking to us.
GM: Yeah, okay, fine.
Shmi: Keep going, I'm picking up a lot of good background here.
GM: <sigh>