Logic

I look young.  I'm used to peoples' initial shock, but most people don't care that much and move on to other more important things.  Other people, it blows thier 'effing mind.  So they keep harping on how young I look.  And that's when I tend to get a little annoyed and/or snarky.
Like I was to one of my coworkers. He sits across for me during a break and opens with:

dude: So, I'm, like, still trying to figure out how you're 29.
me: I was born in 1982. That's how.