This is an experience I've noticed over my tutoring that happens with most students, in general.
If you ever ask someone, "Does that make sense?" they will always, always, always answer "Uh-huh" (or "Yes", or another general colloquial affirmation). I could say "the delta G favors dissociation" to someone memorizing the solubility rules, and they'll only start to nod their heads, look a little confused, but they will answer "Yes" with at least a .99 probability -- I haven't tested that, but I hypothesize that it would happen.
I wouldn't say that I suddenly get the pass on this one, either. If I'm struggling with a concept, I'll often just blurt the first thing that comes to mind to see if it sticks and see if I'm anywhere near the right track. If someone asks if I understand, I'll say "Yes", wait a minute, and then ask a question directly related to what I was just told. Sometimes the answer will be the exact same thing that they just said.
So this got me to thinking about a general possible maxim for teaching: Never ask your students if they understand. Always assume that they do not understand. When they look bored, then that is the point at which they understand.
This isn't always necessary, as sometimes an individual's body language will let you know whether or not they understand the concept. But some people, including myself, are tricky at hiding it... in the hopes that they don't embarrass themselves (at least, that's my personal motivation), and in the hopes that something later will make it all click together.
I am going to start testing this tomorrow.
EDIT: The phrase is a habit. I totally fail.