Riddles in the Dark
This is perhaps the most well-known chapter in the entire book. Bilbo finds the ring, and he rescues himself.
Crawling blindly down a pitch black tunnel, Bilbo puts his hand down on a small metal circle. He puts it into his pocket, and crawls onward.
He meets Gollum, and is challenged to perhaps the most famous riddle contest in literature - to be shown the way out, or to be eaten. Many of the riddles are not particularly unusual and are classic folk riddles, and they solve them fairly rapidly, by turns.
Some, Tolkien makes up himself, and they are unusual and memorable.
Gollum gives Bilbo
It cannot be seen, cannot be felt,
Cannot be heard, cannot be smelt.
It lies behind stars and under hills,
And empty holes it fills.
It comes first and follows after,
Ends life, kills laughter.
Bilbo successfully answers "Dark"
Bilbo tells one Gollum guesses easily, and Gollum challenges him with,
"A live without breath,
As cold as death;
Never thirsty, ever drinking,
All in mail never clinking."
Bilbo is puzzled, and Gollum thinks he has won, when a fish jumps from the little lake and lands on Bilbo, and he realizes the answer - fish.