These two are best mates by the look of it. They are messing around. The dog nibbles the cat and the cat retaliates in a playful way by smacking him in the face. All good fun but the dog makes a face. How do we interpret the facial expression? It looks like he is surprised at being smacked. It is an exaggerated expression. I may be humanising this dog. We tend to do that with our companion animals. In which case it might not be an expression of surprise but something entirely different only known to this dog. The dog is named Navi and her feline roommate is Fireball.
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