There is an infinity of such questions and failure to answer any one of them can produce off-task behaviour. Many children won’t even get started. If you are trying to answer all these questions individually around the room, behavior will deteriorate universally. You should never invite children to start something without running a short whole-class clearing-up activity, based on everyone together answering such questions. Of course, when asked, children will frequently assert that they do understand, that they don’t have any questions, then when you tell them to state, they will put their hands up and start asking. This just seems to be a trade-union rule for pupils-never own up not understanding. So there are better ways of handling Q than by simply asking there are any questions or whether everyone understands; for example, why not have pupils always discuss a new task in pairs, for thirty seconds, and decide if they have any problems? Then they can raise their their and ask. This is now a part of the task setting; no one has begun, everyone is listening, problems are being raised and solved for everybody. And then you can say to the class, “Is there now anything that will stop you working when I stop talking?” Such a routine saves the rather wasting it.