Nutcase is one of the hardest puzzles by Hanayama. It is an interesting combination of bolts and nuts, interlocked with each other. The nuts can go up and down along the length of the bolts, but seemingly there is no way to take them out. The idea is really interesting and the solution is very well thought. The problem with it however is that even if one figures out the trick for separating the pieces, it may still take lots of time and energy to do it. Apart from this issue, Nutcase is a brilliant puzzle and is recommended for your collection.