Gavin can't stop laughing and the children and puppets learn how infectious laughter can be. While Tiga, Gavin and Gertie exchange 'knock-knock' jokes, Dad takes Jason and Jodie to the Trick & Joke Shop. The children decide to play some practical jokes on their backyard friends, but discover that jokes aren?t always funny. All ends well and they eventually find out what gave Gavin the giggles in the first place.