The kids in class didn't have any trouble identifying the expression on Ranger's face in the third photo. He is clearly angry and threatening. Ranger will tolerate the furminator for just so long before he's done. It probably wasn't the kindest course of action to push him to the point of snarling but I really wanted an angry photo.
The second photo was also pretty easy for the kids in class to identify. Ranger is smiling. It was much easier to get a picture of him smiling than of him snarling.
The first photo was much much harder for them to figure out. Before you read further go back and look at it again. What do you think his expression is saying?
Had another look? Good. The kids in the class were about equally divided between thinking it was a happy smile and thinking it was a peeved look. It's a perfect example of why so many people get themselves in trouble with dogs. Some dogs are especially subtle in their expressions. I'm lucky because as a rule Ranger is not very subtle when he's communicating with people. He seems to understand that he needs to be very clear or the humans won't understand.
In that first photo I had been petting Ranger and discovered that he'd romped through a burr patch. I was working out burrs and he was just starting to be annoyed. When I look at the picture I see that his eyes are getting hard, his muzzle is starting to wrinkle and his lip is just beginning to curl. I need to stop what I'm doing or I need to provide some pleasant distraction like a treat. If I just keep working out burrs the next thing I'll see will look like the third photo a full on snarl and if I ignore that my hand is going to wind up in Ranger's teeth.
The difficulty people have in decoding dog expressions is what gets them into trouble and what leads to a bite. It's why I try so hard to educate kids and adults too so that they can read a dog's communications and keep both species safer.
I'll end with a more typical photo of Ranger.