system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why does not drinking enough water cause headaches?
gpt
Dehydration causes your brain to shrink. As your braincase is essentially sealed, this shrinkage causes negative pressure between your brain and skull, which has the effect of pulling on the inside of your skull - like being squeezed, but from the inside. This is what causes the pain.