I would ask them questions like, "So, are you telling me that Jesus can fly?" and "Wait, wait, wait... you're telling me God doesn't ride a sled around the world every Christmas, going through chimneys and bringing kids gifts?"Depends on the question, and to some extent, why the topic has come up...
I once spent a joyful 20 or so minutes explaining my beliefs to a couple of JWs who showed up while we were working in the front yard...I don't think they got more than a word or two in edgewise...