I have to admit that, even though I encourage my students to ask questions, my classes are sometimes a bit too quiet. It's quite difficult to strike the balance between quiet enough to concentrate but not intimidatingly quiet, the sort of quiet where no one dares to speak. Funny to think that I spent the first years of my teaching career trying to stop the pupils talking now I'm trying to get them to talk. My students are always keen to discuss stuff just not physics, the problem might be that there is little room for opinions in physics, you are either right or wrong.

"I think a black hole has no mass".

"well you're wrong"

End of discussion.

Of course we don't necessarily need words to express ourselves, here's some communication of a different kind.