He opens his mouth to say something and turns to face her, a look of frustration and pain poorly-concealed by irritation on his face. Before he can say anything though he stops. Zuko might be able to recognize what's happening from his gaze; he's finally noticed her scar and is looking at it, then realizes he's staring and looks lower but keeps seeing it out of the corner of his eye, then he looks even lower and realizes that he must look like he's staring at her chest, so he looks back up, can't avoid looking at the scar, and finally latches on to looking at one of the ostrich horses to escape. His train of thought has evidently derailed, since he doesn't say anything, only clenching his teeth and displaying a slightly pained look as he mentally kicks himself.