Lady Susan and Finn sit at the table eating breakfast. Finn reaches over to grab Lady Susan by the neck.
Lady Susan: Hugsies!
They embrace awkwardly but fondly. Finn turns his head to look soulfully into his mother's face. Lady Susan thinks that he is about to say something sweet and precious, like how much he loves her.
Finn bursts out laughing, highly amused at himself.
Rinse and repeat and there you have a two-year old version of a knock knock joke.