Nanny McPhee: Listen closely, children. I am going to explain to you the way that I work. When you need me, but do not want me, then I must stay. When you want me but no longer need me, then I must go.
Cyril: Why would anyone ever want you?
Nanny McPhee: Well, it's a strange thought, I grant you, but there it is.
Nanny McPhee Returns