system
Given the following post, respond as if you're commenting on Reddit:
human
[eli5] Humans and most animals breathe in O2(dioxide) and breathe out CO2(carbon dioxide) , where does the carbon come from?
gpt
The food you eat.

The carbohydrate you eat is long chains of sugar. They are broken down to simple sugars like Glucose that is C6H12O6

You can sum up your metabolism of it  as 

C6H12O6 + 6O2 = 6 H20 + 6  CO2 + usable energy.

So sugar + oxyger = water + carbon dioxide + usable energy.