"If only we could hear what they're saying!" said Hermione.
"We can!" said Ron excitedly. "Hang on--damn--"
He dropped a couple more of the boxes he was still clutching as he fumbled with the largest.
"Extendable Ears, look!"
"Fantastic!" said Hermione, as Ron unraveled the long, flesh-colored strings and began to feed them toward the bottom of the door. "Oh, I hope the door isn't Imperturbable--"
"No!" said Ron gleefully. "Listen!"
They put their heads together and listened intently to the ends of the strings, through which Malfoy's voice could be heard loud and clear, as though a radio had been turned on.
Eavesdropping device; one of Weasleys' Wizard Wheezes (see). An Extendable Ear looks like a piece of flesh-coloured string. One end is inserted into a person's ear, then, on the command "Go," the other end wriggles off to where a conversation is taking place. The Extendable Ear makes the distant conversation easily discernable (OP4)
Considering this Weasley product and the way it works, there is a dark irony to Fred getting his ear blasted off in the Battle of the Seven Potters (DH5). -BB