I saw this somewhere on the Internet a few days ago and have since forgotten the source.  But it’s so elegant that I want to spread the meme.  Plus, a post on Friendly Atheist reminded me again.

“What would it take to convince you that you’re wrong?”

That one sentence is all you need to figure out whether someone is capable of being reasonable about any particular subject.  In fact, making people admit to themselves that they’re not willing to entertain any possibility of being wrong, could be enough to break the spell.