Here's how it happens:

* Reaction: 2Ag + S → Ag₂S

* Appearance: Silver sulfide is a black, powdery substance.

* Common Occurrence: This reaction is responsible for the tarnish that forms on silver objects when exposed to air. This is because sulfur is present in trace amounts in the air, and it reacts with silver to form silver sulfide.

Let me know if you'd like to know more about the chemistry of silver sulfide!