How can you gain 20 pounds quickly if you are very skinny and tired of looking anorexic?

You must increase your calorie intake. If you are drinking diet soda, switch to regular. If you are eating lo-cal yogurt and whatnot, try the full-fat varieties. You could supplement your existing diet with nutritional snacks, power bars, and protein shakes. In the gym, switch to anaerobic exercises. In other Words, spend less time on the tread mill and more time with the free weights or Nautilus gear. And do fewer reps at greater resistance, as opposed to many reps at low resistance. This way you can add some muscle mass, not just adipose (fat) tissue. I have two thin sons (21 and 18) the younger particularly is extremely skinny (5'9" and 110 pounds)and they are trying to put on weight. Drinking soda of any variety is not good, it fills you up with quick empty calories, drink milk or fruit juice, more protein, fish, chicken, and lean meats, this will give you what you need for muscle bulding, and you need carbs (bread, pasta, rice etc. )for the energy to perform the exercises. If you like nuts they are also a good form of easily accessible protein.