A bird is kept in a cage outside a window. She often sings at night when all other birds are asleep. One night, a bat visits and asks, "Why do you only sing at night?"
The bird replies, "Last year, while I was singing in the daytime, a bird catcher heard my voice and caught me in his net. Since then, I have never sung during the day."
The bat responds, "But it is no use doing so now. You are already in the cage."