A guy goes into a restaurant/lounge 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."