What are jokes?

Marcus asked us this morning "What are jokes?" We tried to explain, and I gave him the example of "My dog's got no nose" my favourite tame joke. (How does he smell - terrible). Elf emphasised that it is often a play on words. Marcus unfortunately picked up and ran with the "smell" concept - his first joke was "What if eight people all called their dogs pooey gooey? HA HA HA HA HA HA HA HAAA!!!"