Castiel:  It 's Enokian . I believe it 's some sort of riddle . Why is six afraid of seven ? Now ... I assume it 's because ... seven is a prime number . And prime numbers can be intimidating .
Sam Winchester: It 's because seven ate _ nine _ .
Castiel: It 's word play . And the answer is the key like the Doors of Durin in Lord of the Rings .
Sam Winchester: Wait a second . You know about The Lord of the Rings ?
Castiel: I 'm very pop culture savvy now .