Replacing bones isn't the only thing done to our skeletons without our input. Apparently, human bodies are fans of the "less is more" philosophy. We're born with over 300 bones, many of which are made of cartilage, but by the time we reach adulthood, we're left with 206. That's because as we grow, our bodies replace the cartilage with bone and fuse some bones together, giving us our final piece count. Or that's the cover story created by the gnomes who steal your bones at night and sell them on eBay.