system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:how come hitting your head really hard knocks you unconscious?
gpt
Your brain isn't really supported by anything, it just floats in some fluid inside your skull. When you hit your head hard enough, your brain moves due to the inertia of the blow, and can actually hit the bone surrounding it. This can cause bruising, swelling, and even bleeding, all of which can impair function. Loss of consciousness is a sign of concussion, meaning there is brain damage. 

Side note - if someone loses consciousness due to a head injury, they should *always* see a doctor immediately, even if they wake up and feel fine.