These kids deserve medals
There are 10 incredibly smart boys at school: A, B, C, D, E, F, G, H, I and Sam. They run into class laughing at 8:58 am, just two minutes before the playtime ends and are stopped by a stern looking teacher: Mr. Rabbit.
Mr. Rabbit sees that A, B, C, and D have mud on their faces. He, being a teacher who thinks that his viewpoint is always correct and acts only to enforce rules rather than thinking about the world that should be, lashes out at the poor kids.
Silence, he shouts. Nobody will talk. All of you who have mud on your faces, get out of the class. The kids look at each other. Each kid could see whether the other kids had mud on their faces, but could not see his own face. Nobody goes out of the class.
I said, all of you who have mud on your faces, get out of the class
Still, nobody leaves. After trying 5 more times, the bell rings at 9 and Mr. Rabbit exasperatedly yells: I can clearly see that at least one of you kids has mud on his face.
The kid's grin, knowing that their ordeal will be over soon. Sure enough, after a few more times bawling of All of you who have mud on your faces, get out of the class, A, B, C, and D walk out of the class.
Explain how A, B, C, and D knew that they had mud on their faces. What made the kids grin? Everybody knew that there was at least one kid with mud on his face. Support with a logical statement that a kid did not know before Mr. Rabbit's exasperated yell at 9, but that the kid knew right after it.