"First of all I should say that I agree with davy" - Norman Tebbit.

That's usually a method with a time limit, after which if the baby hasn't stopped then there is a problem. My friends little girl is a little on the hyper side and tends to cry when she's tired because she doesn't want to give in and sleep. Small children wont cry for long past tiredness because they haven't got the energy, so if crying persists its because there is a physical problem - usually hunger. After all they can't speak, they have to communicate somehow.