Rat Poison

A box of Rat Poison is The Twins' Object of Desire which, whilst not seeming like much compared to the desires of other characters, represents their willingness to murder to get what they want.

Story
It can be found in the beginning when your looking for Trinkets for the Gift Shop Clerk, in the Mansion, in the last Monk Puzzle, and at the end of the game.

The object is used by the Twins to poison the soup their mother is making for dinner, as they want to go outside and be free away from chores and bedtime.