My Mean Face

So the other night Trinity was doing something he should not have been and I lowered my head to show him that I was serious and told him 'Please dont do that". Well he keeps doing it so I give him "the Look", I call it my mean face, and he gives me the same look without smiling. I could not help but laugh so he starts laughing also. Then he thinks it is a game so everytime I try to be serious with him he just gives me "my mean face" and starts laughing! What am I to do? I have a little monster on my hands!