Lisa's Date with Density a child from Josh Weinstein's school days who everyday would get onto the bus with a piece of toast, which had Vaseline on it. A majority of the story is a reference to the film "Rebel Without a Cause". Lisa remarks that Nelson is "like a riddle wrapped in an enigma wrapped in a vest", a reference to "a riddle wrapped in a mystery inside an enigma"; this was Winston Churchill's opinion of Russia at the outbreak of the World War II. In its original broadcast, "Lisa's Date with Density" finished 63rd in ratings for the week of December 9–15,
The Riddle of the Sphinx (Inside No. 9) directed themselves. Close-up shots of the crossword and the blackboard were filmed later, some on the set of "Private View", which introduced potential continuity problems. The episode was filmed after two days' rehearsals. The script was 32 pages in length, resulting in a first cut that was 38 minutes long. The BBC permitted the final version to be a little longer than the half hour typical of "Inside No. 9", but the production team still needed to lose several minutes in the edit. The final run-time for the episode is 31 minutes and 35 seconds. "The Riddle of the Sphinx"