foul trad anglais Riddles

A wealthy man lives alone in a small cottage. Being partially handicapped, he had everything delivered to his cottage. The mailman was delivering a letter one Thursday when he noticed that the front door was ajar. Through the opening, he could see the man's body lying in a pool of dried blood. When a police officer arrived he surveyed the scene. On the porch were two bottles of warm milk, Monday's newspaper, a catalog, flyers, and unopened mail. The police officer suspects it was foul play. Who does he suspect and why?

Old Mr. Smith was found dead in his study by young Jack. Jack recounted his dismal discovery to Detective Anderson, who happened to be around the scene. "I was walking by Mr. Smith's house when I thought I would just pop in for a visit. I noticed his study light was on and I decided to peek in from the outside to see if he was in there. There was frost on the window, so I had to wipe it away to see inside. That is when I saw his body. So I kicked in the front door to confirm my suspicions of foul play. I called the police immediately afterward." With a clue from Detective Anderson, the police officers immediately arrested Jack for the murder of Mr. Smith. How could the detective tell Jack was lying?