Hank introduces us to ecology -- the study of the rules of engagement for all of us earthlings -- which seeks to explain why the world looks and acts the way it does. The world is crammed with things, both animate and not, that have been interacting with each other all the time, every day, since life on this planet began, and these interactions depend mostly on just two things. Learn what they are as Crash Course Biology takes its final voyage outside the body and into the world.