gain weight

easiest way is to increase your calorie intake, but personally I would start building muscle increase your calorie intake and start maybe going to the gym and building muscle as you defiantly don't want to just put on fat. If you build the muscle you will remain healthy and also bulk out.

Your issue is called, by the muscle weight traning guru's, "hard gainers" and pumping up calories and exercising like others will do nothing for you. I know because I am one of them. I kept the ice craem conpanies in business for years. No amount of food will help with what you want to do. The good news is that their are a few muscle building guys out there that actually understand and have some good suggestion but be very careful, following some of these guys suggestion will make your problem worse. If they are not writing for or have worked with hard gainers do not pay attention to them because you have a unique problem. If you exercise like a regular person you will never gain muscle or weight and will actually reduce your over all size. This guy is very good and understands the problem, he has been training for years and has articles that are free and also has a cheap, paid training program that I will endorse for anyone out there with this problem. He has studied this and can truley help you but do not try to exercise more then what he recommends or you will be back to losing weight. Good luck and happy eating. http://tinyurl.com/omlgsu

Most people would be happy to be lean like you. I really don't have any tips but I can give you 25 kilo's of my butt...Heehe. Just eat as much as you can eat and be happy you're not overweight.

Eat food which have a lot of proteins like meats, eggs, try to eat 3 balanced meals per day and do some musculation 2-3 times per week, eat a lot a lot and sleep a lot.

You should eat well, having a solid breakfast lunch and dinner. Eat healthy foods and exercise daily. Not jogging or riding bikes but do some muscle exercises. Muscles also add on to your weight and make you look good.