Q: What's turtle plus baby?
A: An octopus!
Q: Why does a fin have two fins?
A: So it can swim!
As a parent, you laugh at these jokes because they do not yet revolve around trying to get you to say something inappropriate by repeating certain letters or the names of certain planets. Incidentally, during my adventures as a substitute teacher, I did have a fourth grader who repeatedly asked me to "spell iCup." It took quite a few rounds of me starting of the spelling with "E-Y-E" before the realization dawned that it was possible, just possible, that I had heard this joke before.
I love my four-year-old and his random word humor. :)