One of my Dad (Z"L)'s most repeated jokes:
A Yeshiva HS Principal is walking around the halls during first period, making sure that each class is getting off to a smooth start to the day. As expected, most of the classes are getting off to a smooth start. But when he gets to Rabbi Stein's class he looks through the window and gets the shock of his life - On the Board, Rabbi Stein has written six letters:
POT LSD
Concerned, the Principal bursts into the classroom, and confronts Rabbi Stein - "Rabbi Stein, you can't discusss drugs with this class, it's highly inappropriate!". Rabbi Stein begins to laugh, and responds - 'oh that, we're talking about prayer:
POT = Put On Tefilin
LSD = Let's Start Davening