Can't Hear If You're Shouting

One trick I have heard of but have somehow forgotten is just that... to tell a screaming child that you either can't hear or understand him/her because he/she was shouting. What I do is tell my son usually that, "crying and screaming won't help him get what he wants" or that "Mommy cannot help a boy who's just whining and throwing tantrums". I guess it's the same principle but seeing two friends use it on our friend's child reminded me that I could use the tactic better.