A railroad tycoon is found murdered on a Sunday morning. His wife calls the police, who question the wife and the servants. The police collect the following alibis: the wife was in bed, the butler was polishing silverware, the gardener was trimming trees, the maid was getting the mail, and the cook was preparing breakfast. The police arrest the murderer on the spot. Who did it, and how did the police know?
The maid: there is no mail on Sundays