How do you think the unthinkable?
How do you think the unthinkable? Ithburgth.
This joke is really horrible. For this joke to be funny, the listener has to first realize that the jokester is attempting to mimic a lisp (a speech impediment where they can't say an 's' sound), and then they have to remember that the Titanic (the ship, not the movie), was supposed to be unsinkable, but it was sunk by an iceberg.
The original question (as it is understood with the lisp unknown) is actually an interesting question, and the disappointment of not receiving an answer to that question heightens the lameness of this joke.