A mountain has roots that nobody sees. This riddle is from J.R.R. Tolkien's "The Hobbit." It is the first line of the first riddle Gollum poses Bilbo in the chapter "Riddles in the Dark."
The full riddle is "What has roots as nobody sees, is taller than trees, up, up it goes, and yet never grows?" Bilbo engages Gollum in a battle of riddles in order to secure Gollum's help in escaping the caves in which Bilbo is lost. However, if Bilbo loses the game, then he is to be eaten by Gollum.
Gollum asks Bilbo five riddles, each of which Bilbo answers correctly. Gollum answers Bilbo's first four riddles, but when it is time for Bilbo to ask another he cannot come up with one, which would cause him to forfeit the game. However, while he is fumbling around in his pocket, he feels his ring. He asks himself aloud, "What have I got in my pocket?" Gollum thinks this is Bilbo's fifth riddle. When he can't answer it, Bilbo is declared the winner.
Riddles are common throughout literature and are often used to show the hero's strength of mind as opposed to strength of arm. They are found in modern novels and movies, ancient Greek plays and Germanic fairy tales.