You're probably underestimating your muscles. In fact, almost everyone does. While everyone knows, for instance, that muscles are important for function—activities such as walking, climbing, and lifting—few appreciate just how important muscles are for feeling. If you haven't noticed this mood-muscle connection yourself, take heart; it is only a recent discovery. Surprisingly, the entire scientific community remained in the dark until approximately 2003 (1) when a team of Copenhagen-based researchers reported a remarkable discovery: Muscles at work secrete tiny chemical messengers called myokines that exert powerful effects on organ function, including brain function (2). Through the actions of myokines, muscle tissue communicates directly with the brain about its activity, triggering a cascade of biological responses that improve memory, learning, and mood (see Figure 1 below). This newly discovered mechanism implies that a person engaging in physical activities that build and maintain healthy muscle tissue can expect to enjoy a range of cognitive and mental health benefits. Recent clinical trials show precisely this effect (3). If anyone has ever accused you of being complicated, they really had no idea. Although you can't tell by looking in the mirror, the body you see reflected is comprised of more than 100 trillion cells. Cells are tiny; if you put cells side-by-side in a police lineup, for example, about 200 of them would fit in a single millimeter. But that's just the beginning of the miracle we call you. Every cell in your body is a thriving civilization in itself, populated by hundreds of millions of proteins and other molecules, each possessing a work ethic that would put John Henry to shame. Scaled to our size, your cellular citizens fly around at the speed of fighter jets, each busying themselves completing hundreds or even thousands of life-preserving functions per second. They must maintain this frenzied pace without interruption for you to survive, totaling billions of trillions of precisely performed chemical activities every day.