tops and bottoms

I recently became aware that gay men are either tops or bottoms. As a heterosexual male this was new and fascinating information to me, and raised a few questions.1) Is the top always the top and the bottom always the bottom, or does the bottom get to be the top every once in a while, like maybe on his birthday?2) why would you want to be a bottom, if there is the option to be a top? 3) how does the top and bottom roles play into oral?I hope none of this is offensive as I am completely ignorant to the topic