How do you get into Sans's secret room?
I heard and know about Sans's secret room in Undertale which you can access by doing something relating to a neutral route.
However I've done a neutral, pacifist, genocide, and true pacifist route and still got nothing relating to the room. How exactly do you get the key to enter it from Sans?
Note: You need to have finished a Neutral route at least once to do the following.
While doing a run that is not a Genocide one, you will get to the Corridor where Sans is giving you his judgment near the end of the game. Before talking to him, save at the beginning of the corridor. Then talk to him, and reload the game (do not save, just reload the game before talking to him). Then talk to him again. Repeat this action (talking, reloading without saving) three times, and he'll give you the key to his room.
You'll then be able to open his room.