Question: What causes pain? I mean how do we know something is painful?

Christina Pagel answered on 23 Jun 2013:

Pain is a signal sent from our nerves to our brain in response to an injury or some other problem… They are special nerves that respond only when the body has been damaged beyond some threshold. It’s really good that we can feel pain because we normally respond by removing ourselves from the source of pain or protecting things that are painful – meaning we stop getting damaged further…

In fact, some people are born without being able to feel pain and often they die young because they end up injuring themselves (unknowingly) really badly. It’s a really sad condition – here’s an article about it: http://www.bbc.co.uk/news/magazine-18713585

the other problem with pain is that sometimes nerves can misfunction and send pain signals for no reason – this is bad because there’s no external reason for it and so it’s very hard to stop the pain!