Hi Kahuna,
Harks 98 is pretty much on the ball. Put the knotted string in your mouth and wet the knot thoroughly with saliva and gently chew the knot, turning it over and over in between your teeth. This usually works and helps if you have some fingernails to start to pull the knot apart once you loosen it with the chewing.
Another method is similar but requires no chewing. Simply wet the knot with spit or a little soapy water and place on a hard smooth surface, then gently tap the knot with a ball pein hammer (ie the smooth rounded end of the hammer) until it begins to loosen up. The rest is histoty!
Any knot can be removed with patience and determination. Good luck!
Good winds,