A dead body is found at the base of a high-rise building - another suicide jumper, police suspect.
The case is left to a homicide detective, who begins his investigation on the first floor, searching the room on the same side as the body.
He opens the window, and flips a coin, watching it fall to the ground, satisfied with his search. He travels up to the next floor and does the same thing; repeating the process until all floors have been searched.
When he is done, he tells the rest of his team that it is not a suicide, but a murder instead.
How did he come to that conclusion?
He had to open the window on each floor. If it was a suicide, the window would already be open!