system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do extreme temperatures (hot and cold) make sore muscles feel better?
gpt
Your blood vessels constrict (& this reduce blood flow) when cold & dilate (& this increase blood flow) when warm. Reduced blood flow tends to reduce swelling (useful shortly after an injury or using muscles). Increased blood flow tends to improve healing (useful once the swelling decreases).