[Sam grumbles slightly and shakes his head.] If I explained everything, we'd be here until tomorrow afternoon. The short version? That was a demon and I'm sort of special. Their blood gives me power to kill them. [He makes a face because Sam's really uncomfortable sharing that, but if it gets her moving then so be it.]
Besides, I've never heard of a "chosen one". I mean...I have, but not you or one that's a vampire slayer.