Do you ever cover your mouth with your hand or an object unconsciously when you are embarrassed or deep in thought? Illustrations of people hiding their mouths sometimes don't allow us to see their full expressions, giving off an air of mystery to just what's going on.
As expressive as her eyes are, Yui Hirasawa (from K-On!) still has a look on her face that's a bit tough to read, don't you think?
See if you can suss out just what sort of expressions are being hidden in the illustrations below.