I'm not sure a human can explain how they know a chair is a chair, either. They can come up with a post-hoc rationalisation, but that's not guaranteed to really represent the decision-making process they went through.
At best you get an answer that describes one or more conscious decisions and leaves the unconscious decisions out, such as "it looks a lot like a stool because it's low to the ground and has three legs, but it has a back, so I think it's a chair"; when the real answer is that they have a bunch of pattern-matching visual neurons, and those neurons feed into other neurons that detect more complicated patterns, and the concept of a chair eventually emerges.
At best you get an answer that describes one or more conscious decisions and leaves the unconscious decisions out, such as "it looks a lot like a stool because it's low to the ground and has three legs, but it has a back, so I think it's a chair"; when the real answer is that they have a bunch of pattern-matching visual neurons, and those neurons feed into other neurons that detect more complicated patterns, and the concept of a chair eventually emerges.