system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What exactly are headaches, and what causes them?

What are headaches actually in your head? And what causes them to happen?
gpt
There are many causes, too many to mention.

They can be caused by dehydration, muscle tension in the neck and head, migraines, pinched nerves, toxins, withdrawals, blood vessel contracting, blood vessels expanding................