Read the following excerpt from The Mystery of the Disappearing Thief: A lady in a fine gown and hat stepped out of the carriage and strode up to the door of 221B Baker Street. She sniffled and used a handkerchief to wipe away her tears. Taking a deep breath, she rapped on the door of the world's most famous detective.

Mrs. Hudson answered the door and ushered the visitor to Sherlock Holmes flat. The detective stood tall and confident, gazing at Mrs. Blakely with an inquisitive smile. He invited Mrs. Blakely to enter. Mrs. Hudson offered her some tea. After using the tea to warm her hands, Mrs. Blakely felt capable of explaining her situation. Yesterday afternoon, Mrs. Blakely had arrived home to find a masked individual robbing her house. When the mysterious figure heard her approach, he or she ran farther into the house and disappeared. All that remained was the black mask lying on the ground. Her most prized possession, a massive purple gem, also had disappeared. While Mrs. Blakely knew it would be useless to ask Holmes to retrieve the gem, she wanted him to determine how the thief and gem had disappeared so completely.

Sherlock Holmes listened to the tale and agreed to visit the crime scene. The safe was open and empty, but it was not damaged. There was no visible entry into the room where the safe was kept: the windows were sealed, and the door was not damaged. Mrs. Blakely led Sherlock Holmes farther into the house and stopped in the middle of a hallway of old portraits. There were no windows, and the only door was at the other end of the hallway.

"This spot is where I found the black mask," Mrs. Blakely explained.

Sherlock Holmes inspected the hallway with a wrinkled brow. He walked up and down the hallway. He put his hand on the smooth floor. Then, Sherlock's brow smoothed and a glint entered his eyes. Turning to the wall, he pressed the expensive portrait into the wall.

"What are you doing to my painting?" Mrs. Blakely gasped.

"Calm down, Mrs. Blakely," Holmes replied. "The solution to this mystery is staring you in the face. There is no sign of a break in, so the thief has access to the house. The thief disappeared in this spot without using the far door, so there must be a hiding spot within the walls or floor. There are scratches on the ground near this portrait. I felt a draft when I reached down to test the floor. It's elementary, Mrs. Blakely. The thief is hidden in the wall behind this portrait."

Holmes pushed the portrait harder into the wall. The wall moved, revealing a hiding place. Sherlock Holmes lit a candle. Its light revealed a sleeping maid with a gigantic purple gem clasped in her hands. The mystery of the disappearing thief was solved