In Frindle, what question did Nick ask on the first day of school?

Frindle by Andrew Clemens is a novel about a young boy who delights in creating problems for his teachers. By the time he reaches fifth grade, he has a reputation for being able to stall or derail a class. Mrs. Granger, his fifth-grade teacher, is known for telling students to look up unknown words in the dictionary. Nick decides he's going to ask her a good question to "sidetrack [her] long enough to delay or even wipe out the homework assignment."
He asks,

"Mrs. Granger, you have so many dictionaries in this room, and that huge one especially . . . where did all those words come from? Did they just get copied from other dictionaries? It sure is a big book."

Mrs. Granger responds by actually giving Nick more homework than he would have had otherwise, saying,

"Why, what an interesting question, Nicholas. I could talk about that for hours, I bet." She glanced around the classroom. "Do the rest of you want to know too?" Everyone nodded yes. "Very well then. Nicholas, will you do some research on that subject and give a little oral report to the class? If you find out the answer yourself, it will mean so much more than if I just told you. Please have your report ready for our next class."

Then she assigns the rest of the homework to Nick and the other students.
The question Nick asks forms the basis for the rest of the book. This one assignment—together with Nick's personality—leads him to coin a new word for pen. The word, frindle, propels him to adulthood in several different ways and leads him to see etymology, Mrs. Granger, and himself differently by the end of the book.

Nick was famous in his elementary school for distracting his teachers and wasting class time:

Nick was an expert in asking the delaying question--also known as the teacher-stopper, or the guaranteed-time-waster (Frindle, Chapter 3).

Nick decided to try his skills on his new teacher, Mrs. Granger. Nick soon found out that Mrs. Granger was not one to be fooled.
Attempting to distract Mrs. Granger, Nick pointed out that her classroom was filled with dictionaries. He asked his new teacher where all the words in the dictionary came from. Rather than answer Nick's question, she assigned him an oral report on the topic. Nick had intended to ask his question so that he and his fellow students could avoid doing some of their work. Instead, Nick was assigned even more work by his teacher. He was perplexed by this unusual twist. He was not accustomed to a change in his usual plan.