It starts with a single cell. The first cell splits to become two and the two become four and so on. After just forty-seven doublings, you have ten thousand trillion (10,000,000,000,000,000) cells in your body and are ready to spring forth as a human being. You have no secrets from your cells. They know far more about you than you do. Each one carries a copy of the complete genetic code - the instruction manual for your body - so it knows not only how to do its job but every other job in the body. Never in your life will you have to remind a cell to keep an eye on its adenosine triphosphate levels or to find a place for the extra squirt of folic acid that's just unexpectedly turned up. It will do that for you, and millions more things besides.