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I usually use paracord, but for this video I'm using 3/8" cord so that it's easier for you to see what I'm doing. I used a 1" ball bearing for the center of the fist, but you could use a marble, wood/cork ball, rubber ball, wad of paper/plastic, an overhand or Celtic button knot at the starting end of your rope, etc.
The main trouble spot in making a Monkey's Fist is tightening it up. The key is to gradually tighten it up by working the slack out just a little at a time following it all the way thru the fist. I go thru the fist twice in the video, but with smaller cord, you may have to do this several times as you keep the cord from twisting/tangling/overlapping as you work the knot. You can still see the short end when the fist is finished, just tuck it into the fist where it's not visible anymore. Knot Responsibly.
*Note that besides historically being used to weight the end of a line to throw from ship to ship or shore/dock, a heavily weighted monkey's fist(slungshot) may be considered a weapon and has the potential to cause injury if swung or thrown, and carrying such on your person may be illegal in some locations, so know the law in your area.
A monkey's fist tied with small diameter cord/string and used as a lanyard or fob serves as decoration and/or for retention and does not need an excessively heavy core.
Music is "Far the Days Come" Letter Box