What did Bilbo discover when he fooled the dragon into showing the underside of his belly?

When Bilbo fooled the dragon Smaug into revealing the underside of his belly, he discovered a vulnerable spot where the dragon's scales were missing. This critical weakness allowed Bilbo to realize how Smaug could be defeated, as he later shared this information with the dwarves. Additionally, this encounter showcased Bilbo's cleverness and bravery, solidifying his role as a key player in the quest to reclaim the Lonely Mountain.