The answer to the above question is "Keyhole"! This puzzle is currently the most difficult puzzle I have got so far. I think only "Alcatraz - the puzzle" could be competing against Keyhole, but let's not dive into it.
I got this puzzle from Puzzle Master* and I had the task to make a video solution of it. I got it together with a few other puzzles and I thought to solve this next-to-the-last from the group. However, the man from Puzzle Master told me that making video solution of this puzzle would not be fun. This intrigued me immediately. I opened the puzzle and started thinking how the string could be separated from the wire.
On the one hand, there are two objects hooked on the two ends of the string - one round bead and one cylindrical bead. The wire on the other hand is shaped in the form of a keyhole - one round opening on the top and one narrow long opening on the bottom. The wire is twisted three times around, and the starting position is such that the end with the round bead comes out of the narrow opening, while the end with the cylindrical bead comes out of the round opening. If it was vise versa, then each end of the string could pass through the corresponding opening on the wire with no issues, but at this setup it is not possible to do so. Of course, you can move one of the two ends through the corresponding opening, but this will twist the string through the center part of the wire, thus making a small knot.
Having the idea of moving ends of the string through the openings, I thought this would be the final step of the solution - one of the two ends could be taken out from the wire, thus freeing the whole string. But this would need to position the whole string in such a way that each end is in different side of the keyhole and both come out of the same opening.
During my endeavors to solve this puzzle, I found out some other possibilities for taking the string out - it could also be taken out of the wire by moving it through the windings of the wire (don't know how to call them - what I mean is the space which is between the curves on the upmost or downmost parts of the wire), or by simultaneously putting the string through the openings (this one results in the opposite position of the string to the starting position). But what was the correct thing to strive for? Also, what should the first step of the solution be? Should I move the center of the string, or one of the two ends, what direction should I move it, should I make it pass through the windings of the wire and so on? There seemed to be thousands of possible positions and movements of the string on the wire. What should be done and what - not?
During my initial try, I created some very complicated knots, I also used force to move the string at some points (it was so twisted in itself), I even hooked the string on the outer side of the wire (which I understood later was part of the correct path of the solution) and did many of things. I was even two or three steps away from solving it, but then I must have taken another path and blasted all my chances in solving it quickly. I created two easier versions, in which the wire is twisted only once and twice respectively. But they were way too easy and their solutions could not be incorporated in the solution of this one as I expected.
At one point, I saw the chance to use some force and then free the string for good. I hesitated a few days whether I shall apply this or not, but at the end, after returning few times to the same position, I cheated.
Afterwards, I was determined to put the string back by myself, so I first made a custom string. It was very each to reset it if I got stuck, so I became the fun again. But, as I have cheated, I did not know where I should enter the keyhole, so in few days of struggles, I gave up and looked up the solution. Afterwards, I had to make a video solution of it and wanted to do it in a reasonable time. Otherwise, one or two years would have been enough to find out the solution all by myself. So, no more tries from my side.
To be honest, I quickly apprehended the solution. I had to check it only twice in order to remember it and do it without any trials and errors. Since then, I can solve this puzzle very quickly (in about a minute).
Finally, I think Keyhole is one hell of a puzzle, which is only for experts in wire and string puzzles. It will take you extremely long time to solve it, even with a custom version, which could easily be reset. And it will take a lot more to remember the correct path all by yourself. If you consider buying it, do it only if you feel that you can spent a lot of weeks on a puzzle, at some points even leaving it behind to rest (being you having the rest, not it).
My final ratings on this puzzle are:
Idea: 5 (I don't give it 6, because once or twice I was wondering if this puzzle was first created with the idea and then the creator was looking for a solution to it... I just have not find yet any logic in the keyhole)
Implementation: 4 (I don't give here 6 as well, because it just looks like something very simple - a wire winded few times and a string put somewhere in it)
Experience: 5 (again, not the highest, because at some points it really annoyed me... and I would not recommend it to everyone, but I am happy to have it in my collection)
Difficulty: 6 (I want to meet someone who will debate this one)
Overall: 5
In conclusion, I would say that "Keyhole"is the puzzle which made me keen on string puzzles (be it wire and string or wood and string - I am now awaiting my first wood and string puzzle, but I think I already love this kind of puzzles as well), if I have not yet been keen on them at the time of working on it (it is my fourth wire and string puzzle and I also have solved 3 wire only puzzles before it). It inspired me in creating a few puzzles by myself, and soon I might share a creation of mine, too.
* You can check the puzzle here
I agree, this is a beast. I have not solved mine.
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