第 19 集: 折木奉太郎為證明其理論觀點，破解了一起關於偽鈔的虛構謎題。
Houtarou challenges Eru to come up with a random mystery for him to solve in order to prove that a theory can be created out of anything and that Eru can't rely on Houtarou about everything she ponders. After it's announced on the school's intercom, Eru settles on having Houtarou figure out why students who went to a local stationary store are being called into the staff room. Much to Houtarou's discomfort, Eru takes the "game" too seriously, and she becomes dissatisfied with the results. Remembering a news flyer posted on a bulletin board in the school, Houtarou deduces that students are being called to the staff room in order to help authorities find the source of a counterfeit 10,000 yen bill given to the clerks at the stationary store. The theory seems perfect, and Eru is impressed and speechless. However, both she and Houtarou forget why they came up with the game in the first place. The next morning, Houtarou sees in the newspaper that the culprits of the counterfeiting scheme have been apprehended, confirming the accuracy of Houtarou's mock theory. (Source: Wikipedia)