Riddle # 6. Frosted glass
One day, in winter, John finds his dead friend Jack in the living room of his house. John immediately calls the police, who asks him how he discovered the body; he replies that he was passing by and that he decided to visit his friend. He claims to have knocked on the door for a long time, but no one answered, as light filtered through the frosted window. John then approached the window to blow on it and melt the frost, then saw Jack lay on the floor.
The police then decide to arrest John for Jack’s murder; Why?