This question already has an answer here:
There is a round house with only one room. There is a maid who does everything, a butler who usually gets the mail, an annoying husband, and a lazy wife in the house.
The husband is murdered on a Sunday and on that same Sunday the detective comes. It has to be someone in the house because all the prints on the door are just of the members of the household. He asks each of them what they were doing.
The maid says "I was sweeping a corner."
The wife says "I was doing the dishes." and
the Butler says "I was getting the mail.".
Who killed him?