system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 why does rubbing an impact injury make it feel better? or is this placebo?

knocked my elbow on the doorframe and immediately rubbed it, and thought about how my mum would always tell me to rub an injury. does this actually make it feel better or is it just a distraction?
gpt
Gate Theory!  
Imagine there's a doorway between your brain (which registers the pain) and your elbow. Only so many things can fit  through the doorway at once, right?  
Smaller nerves send pain signals, while larger nerves are responsible for things like touch and pressure signals.  
Activating those larger nerves (by rubbing the area) will block out some of the smaller nerves, "reducing" the pain you feel.  
And as an added bonus, these larger nerves will often trigger the release of endorphins, which do make you feel better, in a way.