I just hope to say the right thing to push them over, but I have noticed it just puts them on the defensive
Ther is someone on utube, but I cant find the link now, who suggests asking questions, almost innocently, rather than an open challange. Its a way of sowing a seed, or even nurturing a doubt that is already there.
I think it was this guy:
http://www.youtube.com/watch?v=yCnan8ODotU&feature=mfu_in_order&list=UL
Good luck.
jlp