How do I gain weight?

I'm just wondering how I can gain weight fast. I'm 21 and I weigh 82 pounds. My dog weighs more than I do. I'm going to the doctor for it, but they're not sure whats going on. I can eat all day and whatever I want and still not gain anything. It's getting so bad that I have no energy throughout the day. I don't exercise at all, except chase my son around or have sex. lol. And I don't exercise because I'm afraid to lose more. So if someone has any comments or answers for me, please let me know. Thank you!

You have an extremely fast metabolism.
Its natural.
I've seen plenty of girls/guys like this.
As long as your eating right, and getting moderate exersise, I don't see the problem.

All I do everyday is sit, I only exercise when I chase my son around the house. I eat all day everyday and my metabolism is fine, there's nothing wrong with it. And when you hit around 75 pounds they stick you in the hospital because you too stick and can die. I weight 82 pounds. Not far. I do everything I can to gain weight, but doctors still don't know what's wrong.

dunno how itl fit in with your life, but do a bit of weights in the gym...I was trying to lose weight so not same prob as you, but I put on a stone in muscle weight in like 1 month...which didnt make me hapy lol, but as I was eating less exercising more etc was def bcos of that..also I have lost weight now I've stopped going!!!
a friend of mine also gets advised to eat muesli for some reason...shes pretty much same as you!
good luck