The police are called to a round house where a man is found dead with a knife wound to the chest. The police investigate and narrow down the suspects to three people: The maid, the wife, and the butcher. When questioned for alibis, the wife said, "I was out buying a dress with my friends, and I found my husband dead upon my return." The butcher said, "I was searching the cupboards for some vegetables to serve for lunch." The maid said, "I was in the other room, dusting the corners." The maid was then arrested. How did the police know it was her?
It was a round house, so there were no corners to dust.