Knock-Knock Jokes

Jacie O’Brien was just four years old when her father lost his life during the 9/11 attacks. 
Not long after this tragedy, her mother Lisa found Jacie having a conversation, seemingly by herself, in an empty room in their family home. 
When she asked about it, Lisa was astonished to hear her daughter say that she had been “talking to Daddy” and that he was telling her knock-knock jokes.
To confirm what her daughter was saying, Lisa showed the little girl a picture of her dad and his co-workers (all of whom also lost their lives that day) and Jacie was able to recite their full names, despite never meeting several of them. 
Jacie also told her mother that she often saw her dad on the swings in the playground when she was out playing with her friends.