In Which Teaching Signs Backfires

Lately Wyatt has been making this sign at me when he gets in trouble for throwing cat food (or the cat) around, or something equally obnoxious.  It looks like the sign for grass, and he has been doing it quite deliberately so I knew it wasn't just random movements.  (By the way, guess what else you get when you google "the sign for grass"?)  I signed it back to him one time, and he nodded (with his whole upper body) in agreement, but I was still clueless.  Then Ava saw him do it and together we figured out he was actually signing grumpy.  Huh.