Mr. Steve lives in a mansion. He is very rich. He's so rich that he has a maid, a cook, a butler, and a lifeguard. Yesterday, Mr. Steve was found dead. The chief detective was coming from Sunday church to investigate. The police were already there. They caught 6 suspects. The maid, the butler, the cook, the lifeguard, the postman, and Mrs. Steve. The maid said she was dusting. The butler said he was playing a board game with Mr. Steve's sons. The cook said he was making dinner. The lifeguard said he was watching Mr. Steve's daughter swim in the pool. The postman said he was delivering mail. Mrs. Steve said she was watching TV. Who is lying? You only have one chance because if you choose incorrectly, you accuse the wrong person and the true killer gets away.
The postman was lying. If the chief detective was coming from church, then you know that the post office doesn't deliver mail on Sunday.