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?