Does Anyone Have Any Idea?.Hōtarō 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 Hōtarō about everything she ponders. After it's announced on the school's intercom, Eru settles on having Hōtarō figure out why students who went to a local stationery store are being called into the staff room...