The Thumb Joke

12-yr-old girl.  Her posture was not as good as it should be, very tight and squeezy, so she had her thumb clenched up to her hand (like a Stop sign).

Me: Hand down. [she straightens her wrist but is still quite tense].  Floppy.  [I tried to open up the gap between her thumb and hand].  What's this?  What's this doing here?
Girl [bewildered]: I... I don't know.... I've always had a thumb there....

We both laughed.  A lot.