The nail knot is a popular fly fishing knot and is used to attach leader butt to fly line or to attach backing to fly line. This knot can also used to make float stoppers. Just substitute a small straw or hard plastic tube in place of fly line and use rod wrapping thread in place of the leader material.
Many anglers suggest that this knot should not be used for big fish due to fact that the knot depends on the outer coating of the fly line for strength.