how do you gain weight?

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I've been eating a lot since i was a kid but my body system just wont give me some extra weight...im underweight for my age and i dont know if its true that its in our genes to be much taller than heavier..or if it's hereditary.i love my body but its not that sexy, im skinny... can you guys help me or give some advice on how to deal with this? i really wanna gain some.