> One thing I find really cool about weight loss is how the mass actually leaves the body. I don't remember the exact process, but ultimately it is mostly excreted via your lungs - all that carbon and oxygen is breathed out as carbon dioxide.

If my memory serves me correctly, when you want to know how many calories a person is burning--and to measure it accurately, instead of just estimating by their level of activity--you do it by measuring how much carbon dioxide they produce. Metabolism is combustion.