system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What the hell is actually causing the aching when I have a headache?
gpt
It depends on what part of your head is hurting.  If it's the forehead, that's usually dehydration or the forehead sinus.  If it's behind the eyes, it's usually the sternocleidomastoid, or the muscle that sticks out on the front of your neck when you turn your head.  If it's on the side of your head, it's the scalenes, or the muscles along the sides of your neck.  If it's in the back of your head, it's usually the levator scapulae, or the muscle that lifts your shoulders up to your ears.