"How do those hats work then? I mean, obviously it's some kind of Invisibility Spell, but it's rather clever to have extended the field of invisibility beyond the boundaries of the charmed object..."
—Description of Headless Hats[src]

Headless Hats are a magical prank item created by Fred and George Weasley for their shop, Weasleys' Wizard Wheezes. They are small hats covered in pink feathers. When worn, the hat and head of the person wearing it disappears.

Although they are good for frightening people, they do not last very long. It works by extending an Invisibility Spell beyond the object itself. This is apparently unusual and advanced magic, as Hermione Granger found it clever and intriguing. The twins charged 2 Galleons for one.