Unconventional Study Group

"I often hear a voice ringing in my ears lately, and only I can hear it!" "Can't anyone else hear you?" "Yes, only I can hear it, but not all the time." "When can I hear it?" "When I was in this classroom, I felt like there was someone in this classroom that we couldn't see!" "Are you sure?" "Me. I'm actually not sure." "So you came to this classroom at night just to find this invisible person?" "Yes! I already have some clues, I think I will find the answer soon!" "Well, this reason is quite novel, but you still have to pay for breaking the door lock." The head of the school security department said seriously.