Sherlock Holmes' sidekick, Dr. Watson, was a medical doctor. When he set up practice he took over the office and clients of a doctor who was retiring from a life-long practice there. This office was on the second floor of a wooden building. There happened to be another Doctor in the same building, but access to his office was by a different set of stairs. By coincidence, the doctor currently there also took over the office and clients from a previous doctor who retired from life-long practice there, about the same time as Watson's predecessor.
The first times Holmes visited Watson's office he instantly congratulated him on having taken over for the far more successful of the two previous doctors. He was right, but Watson was baffled by how he could have known that was the case.
How could Holmes know, with a mere superficial look at the exterior of the two different offices, that Watson's predecessor had been the more popular and sought after doctor?