Tough knots.

Untying a tough knot rarely happens through force. Rather force only makes the knot tighter. 

To untie a tough knot, you have to be patient, observe the different threads which are entangled with each other. 

Figure out which one you can pull at, with little force and see if it makes the knot looser or tighter. 

You keep doing this until one thread gives way. And then another. 

And soon the tough knot is not tough anymore.