This is the time of rapid growth for your child and amongst the other things that are developing, memory is something that is developing at a fast pace. His cognitive abilities and thinking has greatly increased. His ability to acquire memory, store it and recall it at the right time has also enhanced a lot and this helps him tell small jokes, little riddles and recite rhymes that he has heard. Now when you tell a simple joke to him, he can understand it, store it sequentially in his memory and recall it when asked.
What you need to know
Children this age like to make an impression on people and would like to recreate situations that had left an impression on them. So, if he has heard a joke that made him laugh out loud, he would like to recreate the situation again and again. As a result he might keep retelling the same joke.