Bob: If God exists, then prove it.
Sam: What sort of thing were you thinking of?
Bob: I dunno. How about a miracle?
Sam: Miracles aren't very convincing.
Bob: What do you mean?
Sam: God's done all kinds of miracles - floods, plagues, parting the sea, making the sun stand still in the sky, bringing people back to life, the works. Hasn't convinced you yet, has it?
Bob: They don't sound very likely.
Sam: That's kind of the point. Miracles tend to be out of the ordinary.
Bob: But it makes them hard to believe if you weren't there.
Sam: So you'd be more likely to believe a miracle you hadn't seen if it was a bit less, well... miraculous.
