Paul J. Zak: It is quiet and dark. The theater is hushed. James Bond skirts along the edge of a building as his enemy takes aim. Here in the audience, heart rates increase and palms sweat. I know this to be true because instead of enjoying the movie myself, I am measuring the brain activity of a dozen viewers. For me, excitement has a different source: I am watching an amazing neural ballet in which a story line changes the activity of people’s brains.