There’s nothing complicated about a simple nut and bolt pairing, but this isn’t your average nut and bolt. The “Nut Case” puzzle by Hanayama takes the form of an apparently unsolvable problem: two freely twistable nuts trapped on a bolt with a head on either side. There’s more to it than meets the eye, of course, and if you know what you’re doing you can not only free both of the nuts, but also use the whole mechanism to hide something small… More at PM.
Want one. Not sure why, but I do.