Which bone protects the brain?

The skull, which is not one bone but about 22, fused along wavy seams called sutures. The waviness is structural - an interlocking joint distributes a blow far better than a straight join - and the fusion happens after birth, which is the only way a head that size gets through a pelvis.

A newborn's plates are separate and overlap during delivery, leaving the soft spots that close over the first eighteen months or so. That flexibility is also why infant skull fractures behave differently from adult ones, and why a condition where the sutures fuse too early requires surgery: a growing brain in a rigid box has nowhere to go.

The protection is good against impact and poor against acceleration. The brain floats in fluid inside the case, and in a sharp blow it keeps moving after the skull stops and strikes the inside - which is concussion, and why a helmet that prevents a fracture may not prevent a brain injury. That distinction took an unreasonably long time for contact sports to accept, and the recognition of chronic traumatic encephalopathy in former players is what finally forced it.