system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:  Why only one person gets knocked out with a head-to-head headbutt.
gpt
You get a concussion when your brain gets shaken up inside your skull.  So, the cause of a concussion is less about a forceful blow to the skull, but rather a blow that rattles your head.

When you headbutt someone, your head is moving rapidly and firmly.  When you make contact, your head still keeps moving forward from the momentum; it doesn't shake.  However, the fellow that gets hit recoils his head upon impact.  It's the sudden recoil that shakes his brain around, giving him the concussion.  If buddy held his ground, and didn't move his head or body back, the momentum of the impact would come back at you and stop your movement.  That sudden stop of movement could give you the concussion instead.