Detective Riddles / Situation Puzzles

A couple find a man's corpse on the beach. The corpse is under an umbrella and has a knife on his back. There are no signs of violence and no traces of blood. The autopsy reveals that the man died of natural causes.

Michael is in prison. One day, he has luch, goes to prison courtyard for a while and finally visits the library. At dusk, out of the reach of the security cameras, he changes his clothes. He then goes through the door to leave the prison, without being caught.

Paul comes home tired. He goes to his room and turns off the light to sleep. After a while, feeling uneasy, he turns on the light and looks around. He turns the light back off and tries to sleep again. After repeating this several times, he takes a look under his bed and finds a dead body.