Your allegorical left thumb. As in, if you have a high school diploma, you have a left thumb and no one cares, but if you don't have a high school diploma, then everyone asks why the hell you don't have a left thumb.
Ben: Great, now you can get that lawn mowing job!
Ben: Even though I'm making six figures at this engineering job, I actually never got my high school diploma.
Ted: Omg are you serious? How stupid are you that you can't get a high school diploma!