Hi! Well this is where your nervous system comes in! Something like touching a burning hot plate would stimulate the release of molecules which causes a signal to travel along your nerves and into your central nervous system. This includes your spinal cord and brain. Nerves in your arms and legs are part of your peripheral nervous system and these all link into the central system.
What a great question! The reason our bodies have developed to feel pain is so that the nerves can let our brain know that we are being hurt – so using Francesca’s example, touching a hot plate needs to hurt, because otherwise we might not move our hand and then we could do serious damage to ourselves. There are some people who have higher tolerances to pain than other people – this means they can cope with the pain for a little longer, but scientists aren’t sure if they actually feel less pain or if their brains just handle it differently. There are some people though, with a medical condition called ‘congenital insensitivity to pain’ (CIP) that actually cannot feel pain! This might sound great at first, but because they cannot feel pain they have to be very careful, because they can still hurt or damage their bodies.
Also, your brain can react very quickly to pain to prevent you getting injured further. If you touch a hot pan, the pain signal goes to your brain and the brain sends a signal immediately back to the muscles to move your hand without you even having to think about it.