Your brain is probably the most important organ in your body, and consequently the one that needs the most defense. Sure, you've got that hard bone skull, but that won't always be enough. What you need is a second, metal skull to put over your regular skull. Except it won't be shaped like a skull, unless you're an evil villain. Probably it'll look more like a bucket, with holes for your eyes to look out of. But so long as it keeps your brain safe, who cares what it looks like, right?