A guy goes into a restaurant wearing a shirt open at the collar and is met by a bouncer who tells him he must wear a necktie to gain admission. So the guy goes out to his car and he looks around for a necktie and discovers that he just doesn't have one.
He sees a set of jumper cables in his trunk. In desperation he ties these around his neck, manages to fashion a fairly acceptable looking knot and lets the ends dangle free.
He goes back to the restaurant and the bouncer carefully looks him over for a few minutes and then says, "Well, OK, I guess you can come in - just don't start anything."