Tying an overhand knot in the top of the bag seals it so smell won't escape.
From the charlotteobserver.com
The other end is sometimes tied simply in a small, tight, overhand knot to prevent fraying.
From the en.wikipedia.org
First, drape the tie around your neck and make an overhand knot with one side a few inches longer than the other.
From the latimes.com
Hold it flat, centered against the overhand knot.
From the latimes.com
A regular pentagon may be created from just a strip of paper by tying an overhand knot into the strip and carefully flattening the knot by pulling the ends of the paper strip.