Trick or treat?

I've bought a small stash of Dairy Milk chocolate bars and left them near the front door, in preparation for tonight's 'trick or treaters'. They will be handed out to the little monsters - quite literally, of course - who come to our front door, dressed in a range of weird and wonderful costumes. Vampires, witches, ghouls, zombies - you name it. As the masked children call out "Trick or treat!", the parents often stand, somewhat sheepishly, at the end of the path as the treats are handed out. Mind you, I've never called their bluff and retorted "OK, then... trick"!, because you're not quite sure what they might have up their sleeve. Possibly, nothing, but you never know. When I was a young lad, I remember pouring some flour-and-water glue - with food colouring, for added effect - on some people's paths if they refused to give us anything, which I now recall with a mild sense of regret.