There is a man lying dead in his office. He is all alone and the doors and windows are locked from the inside. There are no marks on him. There is no blood. There is a sealed envelope on his desk. How did he die?

“He was murdered by someone outside.”

The riddle specifies that the doors and windows are locked from the inside, indicating that no one could have entered or exited the office.

“He had a heart attack.”

While a heart attack is a possibility, the riddle emphasizes the mysterious circumstances of his death, including the sealed envelope, which suggests foul play rather than a natural cause.

“He committed suicide.”

Similar to the heart attack guess, the riddle implies that there is more to the situation, particularly the sealed envelope, which points towards an external factor rather than self-inflicted harm.