We don't have a lot to work with: a computer and a projector at the front of a big room, often full of computers they can hide behind or surf on. Then there are clickers they can vote with, voices they may or may not choose to use, phones and tablets they may prefer to focus on, friends both inside and outside the room who may distract them. If we're REALLY lucky we may have the real life* or virtual support of a tutor, telling them they need to listen to us. We have our enthusiasm, friendliness and knowledge. They may bring apathy, interest, confusion, enthusiasm, language barriers, disabilities both seen and unseen, immense talent, enormous real life problems we know nothing about...or indeed all of these in just one student.
So that's the challenge. What the hell are we going to do about it?
* This only happened to me once. It was the most enjoyable session I've ever taught.