Many of us remember playing with small plastic versions of the 15 Puzzle when we were young.
The puzzle was invented in upstate New York in 1880 and caused a craze in both the US and Europe.
Many people work out techniques for systematically solving the puzzle. Here in the Brain Gym, the
goal is to solve it as fast as possible with as few moves as possible. Even when you have worked
out techniques for getting to the solution, that takes concentration
that builds up mental muscle.