The start of the week is usually not a good time for puzzling, when the grind of the work week gets underway. Thus I would normally try to find a not too difficult puzzle to play with, so that my chances of solving it within the same evening are very much higher, and I would not feel like crap going into Tuesday. After looking around a bit, I settled on the Hanayama Cast Square which is rated 4 out of 6 stars for difficulty. I couldn't find a Hanayama less than 4 stars.
The Cast Square is the creation of Vesa Timonen, who has designed many of the Cast puzzles available on the market today including the Cast Loop, Cast Cylinder and Cast Donuts. This puzzle came to the market in 2010. It comes in a shiny brass and chrome finish; I would have personally preferred a matt finishing instead.
The object is to separate the 4 odd-shaped curved pieces, all which look rather identical. The 4 pieces can be expanded and pulled apart but up to a point and thereafter, all further movement is restricted. The 4 pieces can be "easily" taken apart but there is a trick to it. You need to spread the pieces apart to a certain point where you can then "release" the square into two halves, absolutely no force needed. I discovered the solution more by accident than anything else; well at least I got it solved!
The solution is actually very elegant as I later discovered and rather easily repeatable. But there is a particular sequence of moves and a technique to it as well. If you look at the pieces carefully (after you have gotten it apart), you will understand why. Putting the Square back together required some fiddling but also quite manageable.
A nice pocket-able puzzle which you can pull out to show friends, and amaze them how quickly you can take it apart when they get stuck.