Answer: Wind
After making an agreement with the creature Gollum, Bilbo played a game of riddles where winning is the difference between life or death. Bilbo was given the first chance to ask. He offered a riddle on teeth. The riddle with the answer "wind" was "voiceless it cries, wingless flutters, toothless bites, mouthless mutters". Bilbo got the answer after seeing the wind slightly move the water in Gollum's cave. In the end, Bilbo left with his life, finishing with the unanswered, "What have I got in my pocket". Remembering when Bilbo first came into Gollum's cave, he picked up a gold ring and put it in this pocket. If only Gollum knew, but then again, he did figure it out after the game when he went to go look for his "birthday present" and found out that it was lost.