Because Bilbo won it in his game of riddles with Gollum.
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In chapter 5; Riddles in the dark; bilbo finds a ring.
Bilbo said, talking to himself, "what have i got in my pocket?" (it was the Ring). Gollum thought it was a riddle and Bilbo decided to make it one. he gave Gollum three chances to guess and gollum lost after four. then gollum went "to get something" (it was the ring), and Bilbo thought he was abandoned and left with the ring on. Gollum thought Bilbo knew the way out and he wanted to follow him, realizing Bilbo had the ring. Bilbo saw Gollum racing towards the gate and followed him to safety.
Bilbo's mother was called Belladonna.
He was so pleased by their praise at the way that he so cleverly snuck into the camp without being seen, that he decided to keep the Ring out of it for a while.
Bilbo escaped from the cave in The Hobbit by using his wits and the magical ring he found earlier in the story. He put on the ring, rendering himself invisible, and slipped past Gollum, the creature guarding the exit. Bilbo then followed Gollum out of the cave and made his way back to the company of dwarves.