Pinball machine

In a pinball machine, a ball bounces off a bumper at an angle of 72 degrees and then bounces off another bumper that is parallel to the first. If the angles that the ball comes to and leaves the first bumper from are equal, what is the angle that the ball bounces off the second bumper ... 

Rutlien. R. Quantum Pinball Machine, Sci. Amer., 38 (November 1991). 

So the trajectories must have a bizarre kind of long-term behavior. Like balls in a pinball machine, they are repelled from one unstable object after another. At the same time, they are confined to a bounded set of zero volume, yet they manage to move on this set forever without intersecting themselves or others. 

Our brains—and our lives—are more complicated than any pinball machine, and we re only beginning to understand all the different factors that create our energy, our well-being, and our mood. In this book, you ll have the opportunity to learn more about some of what I ve come to consider the key factors—brain chemistry, diet, exercise, mental outlook, and openness of spirit. Working with any one of these factors can make a huge difference in overcoming your depression. Working with all of them at once can create a kind of quantum improvement that may, over time, astonish you. This potential for both slow transformation and quantum leaps is to me the true value of the chemistry of joy. 

---