Is it simple or difficult? Depends on how patient you are. The concept in Nogglers NG-002 4 Inch Missing Marble is simple. There is only one hole. Therefore, the marble had to go in there and it has to come back out the same hole. Inside there is a maze. To construct the maze, 5 holes were drilled into the block but you don’t know if they intersected or not. Then 4 of the holes were plugged, leaving one open. Thus a maze was created. So, the challenge here is to move the block around and feeling the ball move until it finds the exit hole. When you finally get the ball out, simply drop it back in, rotate the block a few times, then the challenge starts all over again. Challenge yourself on how quickly you can get it out again. You can purchase this puzzle at Amazon.