"I'm a magic nerd. If I keep you around for three days - or call it six, I suppose - then I have all that time to interrogate you about what you've got back home, and then there's always the possibility you'll turn up with something new and exciting after. Dunno if it'll work on people from other worlds but back home sometimes people have magic, and it sometimes gets better after they turn, and sometimes they don't but they do afterwards. Idiosyncratic per-person powers. Called witchcraft."