Clear Hint

Exploration Stage: Post Office
From: Kirra Woodville
Description: The riddles are left for me by someone who knows that Iflew to the Lower City. But who is it? Under the flask's top there's another note: "What can travel the world while always staying in the corner?" Let's look for a stamp. I'm sure this is the answer!
Completed: This is an envelope with a stamp, and there's an airplane on it. This is a clear hint about my flight in the airship. It was a secret mission: maybe someone has decided to blackmail me with this information?