The human head is quite hard! But what's inside is pretty soft, and we're not just talking physically. There are a lot of people in the world who've made a fool of themselves and ended up with karma, or their own poor judgement, getting the best of them and delivering them with a crisp, clean knockout. The skull is made of a thick layer of bone that protects the brain, with a thin membrane inside. It's usually enough to reduce impact, but the brain sloshing around inside that skull can have a tough time dealing with being crashed about the person carrying that gray-matter. With so many collisions, or severe enough ones, the brain can go through a concussion, or even worse. So be careful kids, and if you're going to do something stupid with your skull, please wear a helmet.