system
Given the following post, respond as if you're commenting on Reddit:
human
How do scientists determine how many calories different activities burn? And how accurate are the estimates on exercise machines?

So I kind of understand how they determine calorie content of food. My understanding is that they burn it and measure the heat and duration, and that gives them the basic estimate. But how do they figure out how many calories the human body burns?
gpt
I think it has to do with measuring the CO2 you exhale when exercising. So you wear a facemask which is connected to a machine which can measure the amount of CO2 you are breathing out and can then estimate the amount of calories that represents while you are running on a treadmill etc.