The Ton-Tongue Toffee is a sweet invented by Fred and George Weasley for Weasleys' Wizard Wheezes that, with the power of the Engorgement Charm, makes the tongue of the consumer swell to an indefinite size (at least four feet long).
In 1994, the twins "accidentally" dropped one on the floor at the Dursley's which Dudley Dursley then picks up and eats. Uncle Vernon was so outraged at this that he flung several pieces of china at Mr. Weasley before letting him restore Dudley's tongue.
The twins tried to sneak some toffees into the Quidditch World Cup, but were caught by Mrs. Weasley's Summoning Charm.