Question: You can find them on plants. One is for music. Most have two. Someone may ask you for yours. Politely, you lend yours to another. Only if you are good you can play by it.
Answer: Ear -- Ear of corn, We say "Ear for music.", We have 2 ears, We say "May I have your ear?", We say "I can lend you my ear.", We say "Play by ear."