The brain sits snugly inside the skull, but it's not a completely flush fit - there is still a layer of fluid between bone and soft tissue that serves as a natural shock absorber. Some shocks, however, can't be absorbed, and when the head gets clobbered too hard, the brain can twist or torque or rattle around inside its skeletal casing.
![The brain sits snugly inside the skull, but it's not a completely flush fit - there is still a layer of fluid between bone and soft tissue that serves as a natural shock absorber. Some shocks, however, can't be](http://cdn.quoteddaily.com/images/jeffrey-kluger/the-brain-sits-snugly-inside-the-skull-but-its-not-a-completely-flush-fit---there-is-still-a.jpg)