How can I gain weight fast?

I wish I had that problem!! Working out builds muscle which therefor increases your body mass in a healthy way. (lifting weights is great for this). Some people are just very skinny naturally, so as long as you are eating healthy and taking care of yourself, be happy with the body you were blessed with. Not everyone is as lucky as you. One more thing, check your BMI (body mass index). You can find a BMI calculator on the web, type it into a search engine like google and click on a link. Your BMI can tell you if you are under weight for your height. If you have always been skinny, I wouldn’t worry about any underlying causes, but if this is s new thing and you have been losing weight rapidly, check with your doctor to make sure there are no health issues causing the weight loss. Good luck!