Tie

A guy goes into a fancy bar wearing a shirt open at the collar, but the bouncer tells him he needs a tie to get in.

The guy doesn't have a tie handy, so goes out to his car and gets his jump leads. 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 bar.  The bouncer looks him up & down and then says, "Well, OK, I guess you can come in - just don't start anything."