The minimum principle for harmonic functions

Solutions to Laplace's equation are called harmonic functions. A non-constant harmonic function can only achieve it's minimum on the boundary of the region it's defined on, which means that if a ball is placed on the graph of such a function, it will always fall off it because there are no minima for the ball to get stuck in. A harmonic function is uniquely defined by the values on the boundary. In the game you control a ball by changing the boundary values. The goal is to collect orbs while trying to make the ball not fall off.