Hi! Do you know "astronauts' jagsaw puzzle"? They are made of pieces with no images, which compell us to try one by one. It is said that this fascinating name is from astoronauts' training menu to maintain his/her concentration.
Yesterday I completed 108 pieces with 3 hours of playtime.
There exists one which is made of 2000pieces, but I didnot feel like to try...
Bye!