Good Riddles

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.

Question: 20 kids go inside the pool and 20 for heads come out

Question: What was the calendar so cramped up and going crazy?

Question: What is brown, has a tail and head?

Question: Without one or the other both would not exist. what i'm I ?