The snail-like object Hermione Granger threw at Harry Potter.
A snail is a mollusk with a coiled shell. Snails were used in Transfiguration class as part of the vanishing curriculum.
Behind the scenes
Also, during the 1993–1994 school year, Hermione Granger threw a possible snail shell into Hagrid's hut, which struck Harry Potter in the back of the head.