Similar to Bishop's riddle below, and mentioned in an irc channel. This riddle comes from an old commercial and is highly amusing.
If you have it, you don't need it.
If you need it, you don't have it.
If you have it, you need more of it.
If you have more of it, you don't need less of it.
You need it to get it.
And you certainly need it to get more of it.
But if you don't already have any of it to begin with,
you can't get any of it to get started with,
so you have no idea how to get any of it in the first place.
It's a bit silly for a riddle, but nonetheless a good one.