It's a bit surprising that the number of degrees burning your food will raise the temperature of water is related to weight gain. I find it more surprising that this simple relation has withstood the test of time.
Can’t tell if sarcastic or not. Burning food to release chemically stored energy therein seems like it would correlate naturally to the chemical energy an organism could extract.
Not at all sarcastic. I mean, we're told this process all the time so it seems like "duh", but if you really think about it there are lots of jumps and hand waving in that process.