There was an episode, maybe the same one Oscarfan is thinking, where Ernie was pretending to be a cowboy for a day, while Rosita is TRYING to teach him various little details of Hispanic lifestyle, but between him and his play horsie, he doesn't pay attention and doesn't listen to her, which really frustrates her.
Matter of fact, I was thinking about a scene from that episode the other day, where they order tacos in Hooper's Store, and Rosita tries to teach Ernie how to properly hold a taco, so that he can eat it, without any of the filling falling out and making a mess, but by the time Rosita finished her demonstration, Ernie and his play horsie finished sharing their taco.