How Do You Make Thin Hair Fuller?

I hate to burst your bubble, but their is no magic hair product that makes thin hair fuller. That is, if your hair is naturally thin, the texture is what it is. Now, keeping it cut shorter gives you a fuller look, but the longer you let it grow the thinner it looks. It's just like looking at two people whose weight are the same in pounds, but one is short and the other is tall. The taller person looks thinner and the shorter person looks heavier because of the difference in length/height although their weight is the same. Your hair strands never change in weight/diameter, it always remain the same. You can have it colored darker to give it an illusion of a fuller look, but it still is what it is...and if it's already dark, learn to appreciate and work with it.