Ton-Tongue Toffee

From Hpwiki

A Ton-Tongue Toffee is a magical sweets - joke sweets invented by the Weasley Twins. They makes your tongue grow longer and longer, and longer ...

Dudley Dursley was the (unfortunate?) victim of a Ton-Tongue Toffee when the Weasley's arrived at Number Four, Privet Drive in the summer before Harry's fourth year to take Harry to The Burrow and thence onto the Quidditch World Cup.