Fuji: "Why would anyone want to make a locked-room anyway?"
Tenkaichi: "That is true. There's no riskier trick than that. Look. [motions to wall covered in pictures, pointing to each in turn] [The murderer hides] behind the door, and when the discoverer [of the body] comes in, and runs away when they are distracted. The murderer who opens the lock and enters first, commits the murder in a split second. The victim locks the door in order to protect the murderer. After the murder, build the wall with the door locked."
Fuji: "They all seem very risky."
Tenkaichi: "Right! Moreover, lately they've run out of ideas, so they just keep on reusing the old tricks. You know, I'm just getting fed up of all these old tricks. Nonetheless, locked-room murders keep on popping up. Why do you think that is?"
Fuji: "Who knows?"
Tenkaichi: "That's because "locked-room murder" is the king of all tricks."