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.