system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does massage work? Like if my calves are swollen and hurting from walking too much, why would pressing on the hurt part feel good or help with healing?
gpt
Your muscle cells burn ATP (a chemical used to store energy) and a byproduct of that is lactic acid. It's waste. Your blood and liver will filter it out eventually, but massaging moves stuff around and accelerates the process. 

Getting rid of waste is generally a good thing.