system
Given the following post, respond as if you're commenting on Reddit:
human
If I had a super precise weighing scale, would my weight change as I breathed in and out?
gpt
Yes, water leaves our lungs as we breathe out, also we breathe in O2 and breathe out CO2.  You would lose weight for both reasons.

It wouldn't however be an up and down cycle, for reasons others have posted.  It would be a slight drop with each exhale.