Truth Telling Teeth

 are a set of false teeth that render their wearer incapable of lying.

History
Some point before the events of the series, the author of 3 comes across and studies the object, recording its powers, appearance and location in said book.

The teeth appear in "Bottomless Pit!" as part of Mabel's story, "Trooth Ache." Mabel grows increasingly sick of and opposed to Stan's incessant lying, and upon finding information on the teeth from Dipper's book, sees them as a solution to her uncle's problem. After retrieving them from the forest, she swaps the truth teeth for Stan's regular dentures in the middle of the night. Mabel is satisfied with the results, though both she and Dipper come to regret that their uncle is unable to filter the truth, often going on to say things no one wants to hear. After Stan's honesty nearly gets him arrested after admitting to several crimes, Mabel is convinced that truth without a filter is a bad thing, removes the teeth from Stan's mouth, and throws them down the bottomless pit in a locked box. She goes on to say that she never saw the box again, though they all see it immediately afterward, falling next to them.

Appearance
The teeth take the shape of large gold colored dentures.