system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How sometimes we can go hours without notice a cut or scratch... But as soon as we do notice it, the pain starts and doesn't go away?

You fall you get cut, and hours later you find blood on your leg and go "oh. How'd that happen?!" Then the pain starts all of a sudden... Why?
gpt
Brain is great at selective listening.  After we put on clothes, we no longer feel their weight or texture.  It's very similar with scratches.  The scratch doesn't register to enough nociceptors (pain nerve receptors) and it's dismissed as minor and unworthy of attention, akin to wearing a heavy coat.  Later, with the notice of blood, the brain acknowledges the damage was bigger than originally thought, and starts listening to those nerves more clearly.

Second part is that it takes a while for damaged tissue to implement the histamine response that increases swelling and pain to the region.  Once that gets going it becomes harder to ignore.