They had to think and react quickly, which is not easy. In a very few minutes they had to select what was important to include and to discard what was not. They had to concentrate on the vocabulary, grammar, etc., and they did it without tension or fear. It was a game, and, as we know, learning through games is spontaneous and natural. There is a wide range of speech patterns within the single genre of joke.
Jokes are common to all cultures.
Jokes embody a culture. Speech behavior that is learned by listening to, and telling jokes can be generalized to speech acts other than the joke. Jokes are funny. They relax the tension in the classroom. What is needed in a classroom is a reaction that is real because it is spontaneous.