Don't worry, I'm around 60 kg. and I'm also 15 years old. You're probably just growing, everybody I know says I eat incredibly much, even more than the average adult, but I still stay skinny. If you just sport a lot then you'll eventually get muscles. 

eat tons and tons of bacon. and when you get fat enough, just exercise and eat turkey bacon to burn off the excess fat (idk, just a thing I thought of one day.)

Scohui : eat a mcdonalds everyday hahaha!

The first thing that came to mind when reading the title of this thread was to eat an huge load of ice cream.

Eat donuts.
Oh.
Don't worry, all the far always goes to your stomach and your legs,cite just how it is.
As you have said, lifting is a very good way of fixing this.
Good luck!

If you wanna gain weight the lazy way, amen to that :3 Eat lots of empty carbs and high fat processed meat, order super sized bigmac meals and never workout at all, if you don't get a heart attack, you'd have gotten fatter and gained weight. Or you could just work out, eat a balanced diet of what you like and healthy foods and enjoy muscles :3

Definitely eat a lot of junk food, and not really do anything beneficial to your diet is a big step to getting fat. Then there's the lazy way and not work out or burn any calories like that.

I'm 5 foot 7 and I only weigh 100 lbs. I am much scrawnier than you are. So trust me when I tell you that you do not need to worry about it. Getting fat is not a good thing. Just work out and build the weight up in muscle instead of fat. Hit the gym, or even just do a bunch of sit ups and push ups every night before bed.

Well, one way to do that is to eat a lot of sugar. Oh, you could eat a crap load of carbs. That is how I gained a couple of pounds. Or, just work out. 

Pretty much what everyone said. Be sure to use weights though, and not body weight. Using real weights will cause you to "bulk up" and become heavier while using just your own body weight will cause you to be toned and slimmer. I just wanted to clear that up.

Fat people does not equal stronger people. Not that simple, kid

You know this is actually a problem that needs to be addressed but not for you lol. It is not a stupid question. I have a friend that cries to herself at night because she isn't able to gain weight and people keep telling her how skinny she is and how great that is but she is dying inside because she feel like everyone is saying that to be nice when everyone really think she is anorexic. That probably isn't the case for you since you are 15. My friend is 23.

You really do not have much to worry for now unless you want to impress the ladies or join a team or something. Right now you are growing, and young people have higher metabolisms. I'm 24 and I have somewhat of a more powerful metabolism than some people my age and I weigh 151 since I last checked 2 hours ago. As others has said, you need to workout with WEIGHTS. Take some whey protein, soy protein, just eat protein. Eat fatty stuff too. Eat a ton of veggies to strengthen your muscles and they will provide you with better health overall. The thing is you already say you eat a lot but that is because of your young metabolism and you are still growing. All that food is going towards you growing. You may think you are not growing but you are 15 so you are. Back in high school a lot of girls would say yeah I'm not growing any taller, I think I stopped growing. That is all bs. You are still growing and that is a good thing even if you are not getting taller. If you want to put on mass, you need to eat even more and lift weights and DO NOT SKIP LEG DAY. You probably do not need a lot of cardio but put that in at least 3 times a week I'd say but you need to do mostly anaerobic exercises which mean without oxygen. If you find yourself being able to do your exercises easily and you breath easily, you are not doing it right. Most bodybuilders would basically tell you this: EAT. and workout. Do the workouts right and do not hurt yourself. Kids just think oh hey let me just bench as much as I can oh yeah I am so cool. Fast forward to when you are 22 and you go wow I could've built muscles much better than the other jersey shores if I just did it the right way. 

Please note that for some people they will never gain that much more weight no matter what they do but once they hit a certain age, it all starts working. This is your metabolism giving you the finger. If your metabolism is truly that powerful then you may just have to wait it out. 

Find a training program, start doing push-ups, pull-ups etc. Eat LOTS of Protein, and keep the same carbohydrate intake. I have the problem that I am small, not unhealthy, but small. I only weigh, 65 kilos, or 131 pounds...

Look at me.....I'm a BLT....I am somewhat healthy with my lettuce and tomato cuts, but parts of me are fatty, like my bacon strips. I try to lose some weight, even decided to...cut off my crust *shudders*, but that was radical for a sandwich like me to do... 

if you ever are blessed with a healthy weight and want to gain more, then just stare at my picture, and my layout

port753 : My abs may have just gotten stronger just by laughing at your post.

As someone who have had this issue himself and still do to some extent this is a very interesting thread for me as well. I'm afraid I don't have too much advice to offer though I did recently start working out using weights which like people said will cause you to bulk up eventually unfortunately I haven't been doing it for very long so I haven't really started noticing the results yet.

While this isn't particularly a proven fact, my sister (who is a chiropractor/health fanatic) claims that one reason why people that are underweight- or in your general region- is because of gluten and dairy. The obvious cases of bread, milk and cheese are affecting your body in some shape or form, but I have no clue as to the science behind it.
It may be something you could look into, however, she also says that it is a 3 month waiting period for it to completely leave your body, so it's a game of patience and commitment that not many will stick to.

Scohui : You definitely do not want to be fatter...that does not mean stronger, It slows you down. I see that you already got it worked out that what you really meant was heavier and stronger. I would at least take up some running or walking so that your legs and stomach do not look disprportionate from the rest of your body. Squats are also a good thing to do. I myself am not very good at situps (i have a weakened, previously herniated stomach muscle) but pushups are also a good idea. I can usually do 25-30 with minimal effort and about 50 if I try a little harder. Eating a ton more is not your solution. Grow muscle, that really helps and eat enough. If you do that, I would think that you should be fine.

Scohui : You might have a super high metabolism. I could eat anything and never gain weight, even when I tried. My body started slowly down in my late 20's, I would say odds are you will get a shock then, but until that time - eat like a roman lord, food on all sides of you!

Well, my mom told me if you eat raisins, you'll gain weight in a healthy way. She's the one who searched it, not me.