Knock-knock?

from an email sent to a cousin....

Last week you mentioned [her son, who shares his name with our Lil'S] saying his first knock-knock joke.  We had a funny coincidence.  Our [Lil'S] has an expressed language delay, but his "word of the week", starting a few days before you emailed just happened to be "nah-nah!" (knock-knock).  His new game was knocking on doors and saying, "nah-nah!"  

Well, on Wednesday, [H] happened to tell some not-quite-funny knock-knock jokes when sitting bored in the car while I was nursing [N] after a visit to the library, so I pulled out a couple real ones, and we had a bit of silly time together.  The next day, when [Lil'S] pointed out pictures of cars (blows raspberries: "b-b-b") or trains ("oo-oo!") in his books, he'd say, "b-b-b-b! nah-nah! b-b-b-b!"  or "oo-oo!  nah-nah! oo-oo!"  I couldn't figure out what he was trying to tell me for a couple days, but then it finally hit me: it was our [Lil'S's] turn for his first knock-knock jokes!  Lol!  After he said "nah-nah", I answered, "who's there?" and he exclaimed, "BBBB!"  thrilled that I finally got it.  ;)  I replied back to him, "Knock-knock!" and he answered something resembling "who's there?" and I said, "Train!" and we just laughed and laughed!