system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What happens when we feel shivers down our back?
gpt
The arrector pili muscle of each hair contract, making the hair stand up. 
It's a vestigial trait from when we were covered in thicker/coarser/longer hair, we would puff up (like cats do) when threatened to look bigger to potential dangers.