The third most important factor is social interaction. Interacting with others is among the most complex things we can do with our brains. It could be through playing music with them, playing bridge or golf, acting in community theater, reminiscing, or discussing literature in a book group. Nearly every part of our brains is activated by interacting with others, live, face-to-face, in real time. (Sorry, Skype.) Doing so requires us to read their body language, the emotions in their faces, and the contours of their speech.
