If it were a suicide, the window on the floor the man jumped from would already be open. Since you had to open every window yourself, it shows that none of the windows were open, meaning he couldn’t have jumped, proving it was a homicide.
The 13-Story Mystery
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?