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Use a pair of needle-nosed pliers, forceps, or even a wrench to grip one side of the knot itself (as opposed to its “arms”).  Hold onto the knot with your other hand or with a second pair of pliers.  Tug gently at the knot.  This is done to loosen up the knot.  Once it loosens, pull the knot apart with your hands. Do not pull as hard as possible or you might break the knotted material. Creating space within the knot will help loosen it. Depending on the nature of the knot, you could use a variety of tools for this. For small, tight knots, try using the fine teeth of a plastic comb or one tong of a fork. On larger knots, you might be able to use a bamboo skewer. Anything pointed and not too sharp will do.  Slip the implement of your choice into the knot.  Wiggle it back and forth a bit. Without removing the implement you worked into the knot, pull at one portion of the knot to see it if it comes apart. If the knot is still too tight to untie, wiggle the implement you placed in it a bit more, or take it out and try to push something with a slightly wider girth into the space you opened up.  Afterwards, try to untie it again. If you can place your knot on a hard, flat surface, you could attempt to untie it using a hard fist-sized rock or a hammer.  Start with a gentle but firm tap using moderate force.  After a few taps, test the knot to see if it can be untied.  If not, continue, adding force gradually each time you check the knot. Be sure that whatever is beneath the knot will not be damaged by the impact.
Take the knot apart with gripping instruments. Work something into the heart of the knot. Tap on the knot.