A man has three envelopes. One contains money, one contains a note, and one is empty. He opens none of them, yet he knows which envelope contains the money. How?
The simplest answer is: he already knows because he put the money in the envelope himself.
The riddle never says the envelopes were mixed up or that their contents were hidden from him. It only tells us that he does not open them now. If he was the person who prepared the envelopes, he would already know exactly what each one contains.
This is a classic assumption puzzle. We tend to add an unstated rule that the man does not know how the envelopes were packed. Once that assumption is removed, no special trick is required.
Did you spot the missing assumption?
