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."
Question: My thickness is no more than a few centimeters, yet my depth can span to infinity. Although mundane, I intimidate most animals. When in my presence, you will inevitably follow me. What am I?
Answer: I'm a mirror.