The teacher said: "If the shepherd1 put twenty sheep out to feed on the grass in a field, five of them jumped a fence, how many sheep would be left?"
"None," called out little Mac. The teacher said: "I am surprised that you can't count correctly. I know that you are good at arithmetic, but you have made such a mistake now."
Little Mac said: "You know arithmetic, teacher, but you don't know sheep. I know that if one sheep jumped, the rest will follow it to do the same."