In St. Louis it is tradition for trick-or-treaters to tell a joke in exchange for candy.  It is believed that the custom was begun by the Irish immigrants that settled the area.  It is not required, of course, but I always ask my visitors if they have one and almost all are prepared. 

So in keeping with tradition...

Why was the Jack-O-Lantern afraid to cross the road?
Because it had no guts.

Enjoy the fun.