Working Late
Riddle: A man is discovered dead sitting at his desk, alone in the locked office. He did not commit suicide and there was no weapons in the room. The only clue is a sealed envelope on the desk in front of him.
How did he die?
Answer: The envelope glue was poisoned and when the man licked the envelope to seal it, he died.
Source: https://www.riddles.com/2295
