Final answer:
Bilbo gets in trouble with Smaug by answering the riddle of the Barrel-rider incorrectly.
Step-by-step explanation:
The riddle that gets Bilbo in trouble with Smaug is the one about the Barrel-rider. In J.R.R. Tolkien's novel 'The Hobbit,' Bilbo and the dwarves are trying to steal treasure from the dragon Smaug, and Bilbo engages Smaug in a game of riddles to distract him. Bilbo unknowingly gives away his identity as the thief when he answers Smaug's riddle 'What has roots as nobody sees, is taller than trees, up, up it goes, and yet never grows?' with 'A mountain's belly, purely based on his earlier mention of his barrel-riding escape from the elves on the river.
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