You adopt a casual pose. "Oh, didn't you know? All messengers wear things like this nowadays."
"They do?" the little boy asks, surprised. "How come the king's messengers don't wear it, then?"
"Because..." you say, searching for an answer. "Because they can't afford it. I'm richer than the king."
The boy eyes you doubtfully, then it seems his mind is made up. "That's it. I'm not letting you in. You've been lying to me. No-one is richer than the king."
He shuts the door on you before you can protest. You were so close to getting in the castle. You knew that that was the wrong thing to say.