" First rule of being a detective,' the Doctor said a he knocked on the door, 'is to observe. Observe the obvious, and observe the not-so-obvious. Observing the not-so-obvious is not as easy as observing the obvious, but if it were easy everyone would be at it. "

― Derek Landy , The Mystery of the Haunted Cottage (Doctor Who 50th Anniversary E-Shorts, #10)