The impossible screw, as YouTuber and inventor Latheman calls it, is a trick screw that screws in but is unable to unscrew. Despite probably not having any practical applications, it presents a great opportunity for a practical joke. And it's not too hard to make. 
Latheman, on his YouTube channel Latheman's crazy machines, says:
It's a small project you can do in your home shop even if you don't have a milling machine or lathe. A hacksaw and a file will do. 
It also makes a nice last minute present.
If you try this I recommend to try it first with a brass screw. It's very important to deburr it carefully. Otherwise, it will be locked in both directions.
If you give this away I recommend to loctite (a German adhesive brand) a small second nut on so that the big nut cannot be moved away from the split point. It will drive anyone nuts who doesn't know the secret.

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