Buttons

A: Daddy your shirt has buttons again today!
Me: Yes, it's a "button-down shirt".
A: Why is it called "button-down" and not "button"?
Me: Good question.  I guess it's because if you start with the top button, you start buttoning down your shirt.
A: But what if you start at the bottom and go up with your buttons.
Me: You're right, then I guess it would be a "button-up" shirt.
A: I think you should just call it a "button shirt".