<human>: Ok, so I was thinking about this the other day. Most adults have a pretty constant weight, which is to say, at the end of every year, they weigh pretty much the same as they did at the beginning. So a 200 pound guy weighs 200 pounds on Jan 1, 2000, he also weighs 200 pounds on Jan 1, 2001. Wouldn't that mean that the food he eats over that period is only enough for him to not gain any weight but not lose any? And then if he dieted and exercised for a few months and went down to 180, couldn't he then go back to his old diet/exercise and maintain that because the old diet exercise is just enough for him not to lose/gain any weight? In reality we see this doesn't work, but why is that the case?

Can you give me the gist of the text in a nutshell?
<bot>: Why is it that the average person isn't constantly gaining weight but if they stop a weight-loss routine they seem to gain back all the weight?