This one works if you're good friends with your neighbor. Once you pass out some candy to your trick-or-treaters, run out the back door to the next house before the kids get there. Answer the door again and watch their surprise as they recognize you. Say, “Didn't I just give you candy? Oh well, take some more, I guess...” If you can, take this joke all the way down the block to mess with those impressionable minds.