system
Given the following post, respond as if you're commenting on Reddit:
human
When I scratch at a pimple on my shoulder, why do I feel pain simultaneously both around the pimple and at a seemingly random spot on my lower back?
gpt
The nerves in your body run in long strands that connect up to the spinal cord at each vertebrae.  The manner in which these nerves are connected together is kind of like the branches of a elm tree, they are connected to a central trunk (the spinal cord) and they branch off into various directions to cover your entire body.  

When you are scratching your shoulder pimple, you are activating a twig that connects to a main branch.  That branch is *mostly* associated with your lower back.  Your brain recognizes that there is a signal coming from your lower back branch and does it's best to interpret that signal.  So the "pain" you feel is not due to an external stimuli on your lower back, but from you brain making its best guess about the errant signal coming from your shoulder.