Muggles' Guide to Harry Potter/Magic/Extendable Ear

Overview
Extendable Ears are inventions of Fred and George Weasley. They look like long, flesh-colored threads; you insert one end into your ear, and can hear what is happening at the other end of the thread.

Extended Description
We first hear that Fred and George have been trying to hear meetings of the Order of the Phoenix using Extendable Ears, and have not been having terribly much in the way of success; their mother had tried to destroy their entire stock but had missed a few, and lately had been placing Imperturbable Charms on the doors to the meeting rooms, so that the ends of the Ears cannot get inside. We see the twins attempting to eavesdrop on some of the order members departing after a meeting, but they leave the entrance hall too quickly.

When Mr. Weasley is in the hospital following the snake attack, he, Mrs. Weasley, Mad-Eye Moody, and Tonks have a quick meeting where they try to figure out how Harry could have perceived the attack. Harry, Fred, George, Ron, and Ginny eavesdrop on this conference using Extendable Ears, and hear that the Order believes that Voldemort may be possessing Harry.

In Harry Potter and the Half-Blood Prince, Harry, Ron, and Hermione trail Draco Malfoy to Borgin and Burkes, a Dark magic store in Knockturn Alley. There, they use Extendable Ears to hear what Draco is saying to the store clerk.

In Harry Potter and the Deathly Hallows, Harry, Ron, and Hermione, who are currently moving from point to point around the countryside, happen to be near another party of Magical refugees. They are unable to hear what this other party is saying because of the protective spells that Hermione has cast around the tent, but Hermione provides Extendable Ears, and they all listen to the conversation of the other group. Presumably, the active end of the Ears is outside the area of the protective spells.

Analysis
In Harry Potter and the Deathly Hallows, it is interesting that the listening ends of the Ears can travel to the area outside the protective spells without being physically pushed there – it seems that Hermione does not even have to leave the tent. It appears that unlike normal threads, the outside ends of the Ears are motile; there is some way to tell them a direction in which to travel to find the thing you are interested in hearing.