system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do people nod their head when they hear music they like?

When people like music you see them nod along with it, are they any particular reason why this occurs?
gpt
Charles Darwin researched nodding for his book The Expression of the Emotions in Man and Animals and found that head nods were fairly (not completely though) universal for agreement. So if something is agreeable it seems we are built to nod. Nice music, nod.