I did Karate when at University, and one of my instructors went on to become a teacher. He too had a pretty rough class, and one day he walked in and the class was all attentive. He thought, "What's up?"
One of the class said, "Sir, do you know karate?" "Yes," answered Dick.
"Can you break a brick, Sir?" asked the student. "Yes," answered Dick.
"Like this one, Sir" asked the student taking a brick out of his schoolbag.
So Dick got a towel, covered the brick with it, held one end of the brick up above the bench and did the special secret thing one does when breaking a brick or a stone, breathed a couple of deep breaths, struck the brick with a knife hand and broke it in two.
Same result, quiet, well behaved class for the rest of the year.
Once we were giving a karate demonstration to a group of elderly people, and because we'd travelled to their rest home we didn't have a very solid anvil to do our stone breaking on. So we'd taken two concrete blocks, the ones with two holes through them. Our instructor set them up in the shape of a T, took a large, flat river stone, covered it with a towel, did the secret thing, and hit it with a knife hand. And it didn't break. So he stood back, took several deep breaths, stepped forward again and held the stone under the towel and hit it again with quite a loud shout. The two concrete blocks both shattered and all the old people cheered. So we cleaned up and took the river stone back to the Dojo where it was kept in a place of honour.
