Extendable Ears are flesh coloured strings which are used for listening to things from a distance. They were created by Fred and George Weasley for Weasleys' Wizard Wheezes, their joke empire, during the summer of 1995 (OP4). To use them, one inserts an end of the Extendable Ear into one's ear and says "Go", causing the other end to go where a conversation is taking place (OP23).
Extendable Ears are rendered useless by the Imperturbable Charm. Molly Weasley uses the Imperturbable Charm on the kitchen in number twelve, Grimmauld Place during the summer of 1995 while members of the Order of the Phoenix hold secret meetings. This prevents the Trio, Fred, George, and Ginny Weasley from eavesdropping with Extendable Ears (OP4).
The Trio, Fred, George, and Ginny use the Extendable Ears during the holidays of 1995 in St. Mungo's to listen to a conversation held by Mr. and Mrs. Weasley, Alastor Moody, and Nymphadora Tonks (OP23).