Knot tying! I wish I had learned it decades a go. There are so many different kinds for specific applications but all you really need to know are 7 basic ones. . Not only will you be able to get the job done when needed, you will be able to recognize and appreciate the work of others when you see it. In the words of Archimedes: Give me a roll of 500 pound paracord long enough, and I shall move the world.