The back of one's head. When someone says something unbelievably stupid, it is often a good idea to show them your normal face. You can proclaim "this is my normal face" and then procede to turn around, or just jump right into the turning around of the head, it's your choice. "Normal" can be applied to mean the back of anything, for example, backhanding someone would be showing them your "normal hand," or whipping out your penis in public could be considered showing someone your "normal ass."
Tim-Billy, you smile all the time.
Billy-No, I don't.
Tim-You're smiling right now!
Billy-I'll show you my normal face! (turns around)