Truth Telling Teeth are a set of false teeth that render their wearer incapable of lying.
At some point before the events of the series, Ford Pines came across and studied the object, recording its powers, appearance, and location in Journal 3. Ford describes the teeth as "a weapon to use against deceivers" (at least ones with no teeth.) As an experiment, he tried to wear them for a day, but found immediately that people didn't like him very much when he was completely honest, accidentally making the mailman cry by telling him that he's abnormally hairy, and was pulled over for speeding and admitted to it, getting an absurd ticket. He then decided to rebury the teeth, believing that honesty is the best policy (except for when it is often not).
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 telling 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 via 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. After telling the story to the others as they fall down the pit, she goes on to say that she never saw the box again, though they all see it immediately afterward, falling next to them.
The teeth take the shape of large, gleaming, gold colored dentures.
- 114. "Bottomless Pit!"