system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What happens when someone gets hit on the head so hard that they pass out?
gpt
Bad things. It is much harder to knock someone out that it appears in the movies, and generally if someone does get knocked out you need to get then to a hospital or they could die.

If you get hit in the head enough to affect how think (you see stars, you can't remember what happened, or you lose consciousness) then that's a sign of a brain injury. The brain hits the side of the skull, or parts of the brain stretch. This causes the nerves in the brain to momentarily stop working and, depending on where it is, you can pass out.

The danger is that the brain injury can lead to bleeding inside the brain or swelling of the brain itself. And if the injury is severe enough that you pass out for more than a few seconds, it's very likely. If this happens the brain will push against the side of the skull, or prevent blood from flowing, which can be fatal (generally after a really nasty headache). This increased intracranial pressure can sometimes be treated with drugs, but sometimes it requires surgery to drain fluid or open the skull.