Very nice. My only suggestions are to (1) add a biology section, for people who haven't quite grok'd how the brain is an organ, and (2) tweak the physics section so that it doesn't lead off with quantum physics, if necessary by making two physics sections. The notion that the world's fiddly bits behave according to mathematical laws is neither obvious nor self-explanatory, and starting to explain this notion by reference to casually observable phenomena (rocks, light, magnets, water) rather than deeply confusing and occasionally controversial phenomena (quarks, neutrinos) is a really good idea.