Dr. Rudy Leibel is the Christopher J. Murphy Memorial Professor of Diabetes Research, Professor of Pediatrics and Medicine at Columbia University Irving Medical Center, and an expert in the genetics of obesity. He explains why humans have adipose tissue, how fat engages in dialog with the brain, and the role genes play in how hungry a person feels and in the ability to resist the temptation of junk food. He also tells the story of how a patient encounter sent him down a path towards discovering and cloning leptin, an adipose-derived hormone responsible for regulating body weight.