Quote from : Doctor Who: Match of the Day Book

Leela: Why are we listening to them? It is a waste of time. The Doctor; It is difficult to know what will be a waste of time until after the time has been wasted, by which time it is too late. So predicting what will be a waste of time is something of a waste of time. Unless it gives you pleasure of course when it probably doesn't count as a waste of time. Leela (yawning): I am sorry I did not hear what you said, Doctor. The Doctor (smiling): That was a waste of time then.