Extendable Ears

Extendable Ears were created by Fred and George Weasley. They are long, flesh-coloured pieces of string which one can insert in one's ear, then shove the other end under (for example) a door, and one will hear the conversation or other noise as clear as if it was mere foot away, unless, that is, the door has been Imperturbed.

The ears were invented sometime prior to Harry Potter's arrival at 12 Grimmauld Place in 1995. Upon learning of their existence, Molly Weasley was outraged and binned as many of them as she could find. A number of the ears, however, survived this purge and remained in use.

Known users
The twins' Extendable Ears proved very useful to Harry Potter when he was trying to listen in on the Order of the Phoenix's conversations, and trying to figure out what the Death Eaters were doing. Harry also used them in 1996 when he was trying to listen to the conversation between Draco Malfoy and Mr. Borgin.

Hermione packed a few Extendable Ears when she, Ron, and Harry went to search for Voldemort's Horcruxes in 1997. The trio used them at one point to listen to a group of Muggle-born fugitives, including Dean Thomas, that had briefly stopped outside their tent while they were on the run from horrible Snatchers.

The use of Extendable Ears during lessons at Hogwarts School of Witchcraft and Wizardry was prohibited, with infractor students being subject to reprimand.

Behind the scenes

 * The books describe them as long and string-like, while in the film adaptation of Harry Potter and the Order of the Phoenix, there are human-looking ears on the distant end. Crookshanks interfered with one of them, knocking it off the "string."
 * The Wizarding World of Harry Potter sells extendable ears, as a novelty electronic toy. It amplifies sound, from up to 12 feet away.

Appearances

 * Harry Potter and the Order of the Phoenix
 * Harry Potter and the Order of the Phoenix (film)
 * Harry Potter and the Order of the Phoenix (video game)
 * Harry Potter and the Half-Blood Prince
 * Harry Potter and the Deathly Hallows
 * The Wizarding World of Harry Potter
 * Pottermore