The wife killed her husband with a knife in the car. 

Nobody was around to see that she murdered him.

She then threw his body out of the car into the lake.

She was cautious not to leave any fingerprints and made sure no one was watching.

Nobody was around to see that she murdered him.

Wife then threw the knife off a cliff where it was never to be found and went home.

A couple of hours later, police called her and told her that her husband has been murdered.

As soon as she arrived, the police arrested her. How do they know she was the killer ?

    ANSWER The police never told the wife the location of the crime scene.  How did she know? This is why she was arrested.