For more than thirty years-half his life-he was obsessively devoted to investigating Everything in the natural world. Nothing escaped his interest-how our eyes see, why the sky is blue, what forces build mountains, how light travels, where water comes from, and-most fascinating of all to Leonardo-the inner workings of the human body. Nothing stopped him. It was illegal to dissect human corpses, so he did autopsies in secret, even devising a clever way to slice through eyeballs (notoriously squishy!).