"Oh - sorry, Neville! ... I forgot - it was the custard..."
"... creams we hexed -"
-- Fred and George Weasley (GF21)
The Canary Transfiguration Hex temporarily changes someone into a giant canary. After a few moments, the Transfigured person molts back into themselves.
References from the canon
- Fred and George placed this hex on seemingly innocent custard creams so that whomever ate one was changed temporarily into a huge canary. Neville ate one of these Canary Creams, much to the amusement of everyone in the Gryffindor common room (GF21).
From the Web
"Canary Cream" from Harry Potter Wiki