What happened when Mr. Crocker Harris made one of his classical jokes?

When Crocker Harris made one of his classical jokes, none of the students understood it and so nobody laughed. But, Taplow knew that Mr. Crocker Harris had meant it as funny so out of common politeness, he laughed at the joke. The teacher was happy that Taplow had understood the joke while others didn’t. So, he told him to explain the joke to the class. It was a difficult task for Taplow because he himself had not understood it.