阅读短文，并根据其内容选择正确的答案。

Tommy is walking homeby moonlight. He feels another little boy is coming behind him. The boy stops a minute to listen, looks here and there and then he says to himself, "Tommy, run quickly." He runs quickly and gets to Mr. Green's garden. He still feels a boy running after him. Tommy runs home quickly. He goes in and closes the door. He turns on the light. He looks out of the window and doesn't see any body. But he knows that the little boy always comes back after him when he goes out in the evening by moonlight.

Can you guess what it is?