Why am I not gaining any weight?!

My dad says I need to gain weight, because I'm very thin, but I already eat like a pig in every sense of the word. I eat so much food! I sometimes eat my friends' lunches when they aren't hungry, but I never seem to gain any weight.
I participate in sports at school, but I only play two: track (which I have twice a week) and baseball, which doesn't start until March.
Why am I not gaining weight.? How can I gain weight.?
I'm sixteen, and five foot sevenHealth Question & Answer

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