How to Gain Weight

The market is filled with a gazillion set of products, diets, and exercise routines to lose weight, but there is little information for the slender/slim/skinny people who wish to gain weight. It is obvious that to have a good shape one must retain a balance, and too fat or too skinny is never good. So how does one go around to actually gaining weight? The obvious answer that would pop to one's mind is: "well I just need to eat more, no?" Though this is true, it is not at all as obvious as it may sound; if the issue was just 'eating more' then people who wish to gain weight will just EAT more. The analogy here is very similar to the adverse, obese people would ask, how do I lose weight? To which the answer could be: "Well you just have to eat less". Theoretically it should be simpler to eat less than eat more, but many things are not what they seem to be. If people could just eat less or eat more, there wouldn't be a need for strict diets, and strict exercise routines.