Silver Key

Found in the dresser drawer of Sans' room. Used to unlock Sans' secret room on the backside of his house containing clues to his past such as a broken machine (time machine??), photos from his past, and blue prints written in an undecipherable language.

The Silver Key can only be acquired in a Pacifist Route once you've spoken to Sans in the hall to the throne room. Save, reload and talk to him again until his trust is gained and he gives you the key to his room.