system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does exercise boost energy levels?
gpt
Regular exercise makes your muscles and your heart stronger. When you're stronger, it takes less effort to finish your regular day-to-day tasks. This makes it seem like you have more energy leftover after doing your regular tasks. This goes for intentional physical activities as well as just being alive (I.e. having a beating heart). 

It may help to think of this in reverse. If you're in great shape, you get used to doing a lot during the day. If you were suddenly out of shape, you'd have difficulty keeping up with your former, fit self.