Overview: In and out, in and out. Staying alive is about doing things. This very second, your cells are combusting glucose molecules with oxygen to make energy available, which keeps you alive for another precious moment. To get the oxygen to your cells you are breathing. Breathing is an answer to a very hard problem: How do you get the resources that your cells need to survive, from the outside, to the inside of your cells? Every living thing has to solve this problem and the solution is surprisingly different depending on one of the most important regulators of life: Size.