The Professor’s favorite topic is prime numbers, and the narrator and Root also grow to love the secrets they hold. The Professor inevitably repeats himself, but the narrator and Root make a serious agreement never to let him know it, both out of respect and to avoid confusing him.

One evening, the Professor asks them what they notice about the prime numbers between zero and 100. Root notices that two is the only even prime number, while the narrator sees that sometimes they come in “pairs,” like 17 and 19, which the Professor tells them are called twin primes. As the numbers get larger, both primes and twin primes because less frequent—because of that, we don’t know if twin primes are also infinite. The narrator marvels at the Professor’s willingness to admit when he doesn’t know something.

The Professor hates crowds, which is why he dislikes leaving the house: “There was something fundamentally incompatible […] between crushing, random crowds and pure mathematical beauty” (64). He doesn’t need silence, but he does require peace in his day-to-day life. He finds all sorts of things peaceful: completed mathematical proofs, the narrator’s dumpling-making process, etc.

On May 6, the narrator arrives to a leaky sink and a growing puddle in the house; moreover, the Professor seems to have more trouble than usual understanding who she is.