Little pitchers, big ears

Actual scene tonight as Thomas was about to grab one of Trailhead Kid's toys:

Trailhead Kid: "Damn it, Thomas, don't even think about it!"

Me: TK --! [Turns head and begins to stifle gales of uproarious laughter as Mr. T shoots me a dirty look, then struggles for his own composure.]

Why is it the kid doesn't listen to either of us until we say something like "Damn it, Thomas, don't even think about it"?