My daughter's friends eat non-stop junkfood and don't excersise, but they're very thin. Why?

Energy Balance. Ultimately, an individual's weight reflects the difference between calories consumed and those expended. They may eat non-stop junkfood, but if their total calorie consumption is not high, then they can remain thin. In addition, different people have different rates of burning calories at rest, known as basal metabolic rate. If theirs is high, they may burn many calories without exercising.