system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: When you have an itch on your back and someone scratches it for you, why does it then travel to other parts of your back?
gpt
Scientists have known for decades that scratching creates a mild amount of pain in the skin.

Scientists worked out that scratching causes a sensation of pain. This causes the brain to release serotonin is to control the pain. But serotonin activates itch-transmitting cells, making the itch worse

In humans, it is not possible to block serotonin to control itching as it is important for pain-control

Senior investigator Doctor Zhou-Feng Chen, director of Washington University's Centre for the Study of Itch, said that pain can interfere with itching - at least temporarily - by getting nerve cells in the spinal cord to carry pain signals to the brain instead of itch signals.

Dr Chen said: 'The problem is that when the brain gets those pain signals, it responds by producing the neurotransmitter serotonin to help control that pain.'

As serotonin spreads from the brain into the spinal cord, the chemical can 'jump the tracks' – moving from neurons that sense pain to nerve cells that influence the intensity of an itch, she added.

EDIT: Explanation to a 5 year old - It's all in the mind.