“I have always understood,” said Bilbo in a frightened squeak, “that dragons were softer underneath, especially in the region of the—er—chest; but doubtless one so fortified has thought of that.”
The dragon stopped short in his boasting. “Your information is antiquated,” he snapped. “I am armoured above and below with iron scales and hard gems. No blade can pierce me.”
“I might have guessed it,” said Bilbo. “Truly there can no-where be found the equal of Lord Smaug the Impenetrable. What magnificence to possess a waistcoat of fine diamonds!”

“Yes, it is rare and wonderful, indeed,” said Smaug absurdly pleased. He did not know that the hobbit had already caught a glimpse of his peculiar under- covering on his previous visit, and was itching for a closer view for reasons of his own. The dragon rolled over. “Look!” he said. “What do you say to that?”
“Dazzlingly marvellous! Perfect! Flawless! Staggering!” exclaimed Bilbo aloud, but what he thought inside was: “Old fool! Why, there is a large patch in the hollow of his left breast as bare as a snail out of its shell!”

Chapter 12 Inside Information