I find it so interesting that throughout my school years, I used to complain to my friends about being too skinny. No matter what I ate, I just couldn't gain a pound. I was a steady 100-105 lbs during high school. I actually would get turned away from blood drives because they would only take your blood if you were at least 115 lbs.
(Back when you actually wore a dress to prom and not a handkerchief. Look how thin those arms were. I wasn't sickly, but I was definitely TOO thin.)