You can tell a lot about a student by one thing. Where their eyes are looking.
There are so many times I'll explain a concept, then notice that my student is looking in the wrong place in the music. This isn't a shortcoming, by any means.
But if I were so wrapped up in my explaining and didn't think to look at their eyes, so much time would be wasted.
We all look in the wrong place from time to time. It's the human condition.
Now, eyes looking at the windows, the walls, and the clock...
That's a whole other blog post.