Who ate my weed?

I grabbed this red hartebeest red-handed, like when you catch a child eating the last bit of cake that is left in the fridge, than when you ask him: Who ate the cake? He answers that it was not him with a face full of chocolate. 
 
 The expression is what I find most fascinating, the scared face, with the ears up, the wide eyes and the little piece of cake, or rather of grass in the mouth. Obviously he did not hear me approach until it was too late, in other words, it would have been a lion and the story would have been, most likely, different.