Here's how it went:
1. The Riddle: The door was hidden behind a wall, only visible to those who knew the secret phrase.
2. Bilbo's Approach: Bilbo, being naturally curious and resourceful, started by looking for clues. He noticed the wall had "small, round holes" and a "round knob" that looked like a door handle.
3. Asking Questions: Bilbo asks the trolls, who are guarding the entrance, "Is it safe to go through that door, or are there any nasty things behind it?"
4. The Answer: The trolls, being rather stupid and easily tricked, answer that it is a perfectly safe door. They even add that there are "a lot of nasty things outside". This gives Bilbo the key.
5. Solution: Bilbo realized that the riddle was simple: "It's safe to go through this door, but dangerous to stay outside". He said this phrase, and the door opened.
Bilbo's success here highlights his key strengths: his curiosity, his willingness to ask questions, and his ability to think creatively and find solutions.