After publication, Juster sent a copy of the book to the Ford Foundation, with an explanation of how the projected book on cities had transformed into "The Phantom Tollbooth". He never heard back from them, and learned years later that they were delighted by the turn of events. With the book having become an unexpected hit, Juster found himself answering letters from young readers, and a few parents. He found that children understood the wordplay at different ages, and heard from the occasional college student as well. Some students wrote a second time after a gap of years "and they'll talk to me about a whole different book, normally. But now they've got a lot more of the words right. A lot more of the fun kind of crazy references". He learned too that readers were capable of more than he had intended, as in the case of the letter sent by the Mathemagician to King Azaz. Composed entirely of numbers, some readers assumed it was a code and set about breaking it, only to appeal to Juster for help when they were not successful. The numbers were not intended to have any meaning, and were meant to convey that the Mathemagician's letter could not have been understood by Azaz or his advisers.