A man was found dead next to a 13 story building. The police say it was a suicide, but you say it was a homicide (someone killed him).
To prove this, you go to each floor on the building, open the window, and toss a penny out. You do this to each floor until you reach the 13th floor, open the window, and toss a penny out.
How does this prove it wasn’t a suicide?
If the man had committed suicide, the window that he jumped from should have been left opened. But all the windows facing the dead body were closed. This means that someone closed the window after the man died.