Walk-on-spheres method

The Walk-on-spheres method is a way of finding the answer to a problem by rolling balls down an inclined plane. Imagine if you wanted to find out the fastest way down a hill. You could place a bunch of balls down the hill, and then measure which ball goes the fastest and that would be the fastest path down the hill. The Walk-on-spheres method is like this, but instead of rolling a bunch of balls down the hill all at once, you start with a single ball and then keep adding more balls one at a time. This way, you can find the path that all the balls take that goes the fastest.