It was his smell. Smaug had never encountered a hobbit before. He did not recognize the smell of him, but could tell that he had been with the dwarves.
Smaug has never smelled anything like Bilbo and he cannot place it.
After he gets away from Smaug, Bilbo makes up a saying that would become a favorite of his. Never laugh at live dragons, Bilbo, you fool.
because he tells smaug he is Barrel-rider
Yes AND no. Yes because when Thorin, Bilbo and the others went into his lair, he deliberately laid on his stomach on his treasure. Why else; to defend this weak spot! No because he forgot. Bilbo tricked Smaug twice into revealing it. The first was during his conversation with Bilbo. Smaug was very polite with Bilbo listening to his riddles. Convinced that Bilbo did not mean him any harm, Smaug rolled over on his back. Bilbo saw the spot where the scale was missing somewhere on Smaug's chest. The second time was when Smaug realised that Bilbo ran away with a golden two handled cup. This pushed Smaug's infuriation to its boiling point. He started destroying things and eating the dwarfs ponies and when he got to Long Lake, he was flying so high that the chink in Smaug's armor was visible. Bard the Bowman shot him in that very spot, Smaug didn't see it coming.
* Smaug didn't like them thumping their barrels. * Bilbo in being clever about who he was claimed to be 'barrel rider' in reference to his journey down the river in a barrel. Smaug took this to mean he was one of the Lake-men and vowed revenge for Bilbo's intrusion into his lair.
Smaug has never smelled anything like Bilbo and he cannot place it.
Bilbo Baggins
After he gets away from Smaug, Bilbo makes up a saying that would become a favorite of his. Never laugh at live dragons, Bilbo, you fool.
because he tells smaug he is Barrel-rider
Yes AND no. Yes because when Thorin, Bilbo and the others went into his lair, he deliberately laid on his stomach on his treasure. Why else; to defend this weak spot! No because he forgot. Bilbo tricked Smaug twice into revealing it. The first was during his conversation with Bilbo. Smaug was very polite with Bilbo listening to his riddles. Convinced that Bilbo did not mean him any harm, Smaug rolled over on his back. Bilbo saw the spot where the scale was missing somewhere on Smaug's chest. The second time was when Smaug realised that Bilbo ran away with a golden two handled cup. This pushed Smaug's infuriation to its boiling point. He started destroying things and eating the dwarfs ponies and when he got to Long Lake, he was flying so high that the chink in Smaug's armor was visible. Bard the Bowman shot him in that very spot, Smaug didn't see it coming.
Bilbo told Smaug a riddle to keep him distracted.
* Smaug didn't like them thumping their barrels. * Bilbo in being clever about who he was claimed to be 'barrel rider' in reference to his journey down the river in a barrel. Smaug took this to mean he was one of the Lake-men and vowed revenge for Bilbo's intrusion into his lair.
Yes. He tells them that Smaug's underbelly has a small patch where there are no scales. Basically his weak spot.
Bilbo complements Smaug with names to show his power, such as Smaug the magnificent, he also gives the dragon fake names instead of Bilbos real name because the dragons were convincing and tried their best to get information from the speaker if they had decided to let them live for a while longer
In the beginning it was Smaug , then in the end of The Hobbit , Bilbo gains it
Smaug tells Bilbo not to trust dwarves:In return for the excellent meal [of the Company's ponies] I will give you one piece of advice for your good: don't have more to do with dwarves than you can help!Smaug-The Hobbit: "Inside Information"
He saw a small bare patch. That bit of information is what led to Smaug's death.