system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does our weight fluctuate so much during the day?
gpt
You are constantly adding and taking away.  You drink fluids and eat foods.  You urinate and defecate.  You exhale water vapor.  If you drink 16 oz of water, you just gained a pound.

You are building and breaking down muscle and fat as well.