Good Riddles

Question: A woman is in the hospital for a routine procedure. The doctor leaves the room locking the door before he goes, the woman is alone. The doctor comes back to find the woman has died and there is another person is in the room. The door was still locked.
How is this possible?

Question: I give you a family of three:

One is sits down, and will never get up.
The second eats as much as is given to him, yet is always hungry.
The third goes away and will never return.

What are they?

Question: What do you call a sad bird?

Question: You can rest on my pad without touching the keys.
I will sing all month long as long as you pay your fees.

What am I?

Question: A father's child. A mother's child. no one's son.

Who am I?