system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does lifting something heavy build muscle?
gpt
ELI5:

Your body picks up a heavy thing repeatedly and muscles tear under the stress.

Your body recognizes that you now pick up that heavy thing repeatedly to stay alive, it dedicates resources to regrowing those muscles that were torn and build enough new ones to handle picking up that weight so you don't die.

Repeat.