Silver Key

The Silver Key is an object found in the dresser drawer of Sans's room. It is used to unlock Sans's workshop, located in the back side of his house. Inside there's clues to his past, such as photos from his past, a broken machine, and undecipherable blueprints.

The Silver Key can only be acquired in the Pacifist Route, as entering Sans's room is needed to obtain it. To get the key to his room, save, reload and talk to him. Repeat the process several times.

Trivia

 * The fact that this key is explicitly silver may allude to H.P. Lovecraft's short story, "The Silver Key", which focuses on time travel.
 * The cause of the blueprints' illegibility is stated to be "maybe just handwriting"; this might be connected to W. D. Gaster, as the Wingdings font doesn't look like Latin characters.
 * The blueprints in the workshop might be those of the DT Extractor in the True Lab.