Mrs Multravers tested intelligence of humans, to select victims with suitable brains, by first asking them a question about prime numbers. Strax had no problem deducing that 13 would be the fifth number in her sequence, but he failed on the second question. Those who passed all three had their brains extracted. (AUDIO: The Haunting)
Behind the scenes Edit
A prime number is a positive integer that has exactly two divisors: 1 and the number itself.
Generally, 1 is considered neither prime nor composite. It would certainly have been considered a prime, however, in the story's time period of the 1890s. In particular, the entry for Number published in 1890 in the 9th edition of Encyclopædia Britannica stated that every positive number was either prime or composite, and explicitly listed 1 as prime.