system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: On a cellular level, how does exercise make us healthy or improve our health?
gpt
There's also a theory that a lot of cellular damage is caused by slightly malfunctioning mitochondria producing more free radicals (hungry molecules that want to bind with other stuff, damaging it in the process...like your DNA or other cellular machinery) as they generate energy. 

Exercise may force these damaged mitochondria to work harder than they would normally and "kill" themselves in the effort, removing them from the cell, letting healthy mitochondria reproduce to take their place and reducing the number of free radicals floating around.