Used to attach a rope to a log, or where security is not an issue. This knot tightens under strain, but comes undone extremely easily when the rope is slack.
Wrap the rope around the log, then pass the running end around the standing part of the rope. Finally twist the running end around itself three or four times. (Note: this is only shown twice in the animation.)

Tip: Jolly useful for dragging logs back to the camp fire!