The grinding of the teeth in Edgar Allen Poe's "Hop-Frog, or The Eight Chained ourang-Outangs" is an auditory signal given by the protagonist, Hop-Frog, before he is about to play a revengeful trick.  The King and his seven ministers are devilishly rude to Hop-Frog and Trippetta, their slaves; but, once the joke has gone too far and Hop-Frog has had enough of their joking and abuse, he grinds his teeth. At the first grinding, Hop-Frog denies ownership in making the sound because he is still in the planning mode of revenge.  When the final revengeful trick is at its best, Hop-Frog grinds his teeth again. Poe describes it as "a low, harsh, grating sound." After the second grinding, the readaer is presented with the following information: "It came from the fang-like teeth of the dwarf, who ground them and gnashed them as he foamed at the mouth, and glared, with an expression of maniacal rage. . ."