An example labyrinth

As a kid I had the chance to play a game, whose goal was to lead two small metal balls through a maze. This was done by holding the board in hands and tilting the closed surface slightly, so that gravity can move the balls in the right direction. It required a lot of concentration since small changes in the board position greatly influenced the behavior of the balls. Many obstacles and dead ends limited progress along the way. Once I tried to move only one ball, I was mostly happy with its direction. But the second ball always found a reason not to follow my instructions and diverged further from the right path. At best I had mixed feelings, since the better I was doing with the one ball, the worse I was with the other. I did not know what exactly to do—move the ball on the right path to the end, but then risk it to not stay there while I was moving the second or choose to improve the position of the second at the expense of the first. Switching my attention between the balls was exhausting and often led to suboptimal positions for both of them. This was sometimes frustrating to the point that I chose to simply shake the board, hoping that the balls will magically find their way. (Yes, kids do it.) But then they got stuck in the most narrow places in the maze which required even more shaking for them to be released.