This is a riddle posed by a dragon in the short story Maybe Just Go Up There and Talk to It by Scott Lynch. The dragon does not give the answer to the riddle and it is entirely possible that it has no answer and the author is just toying with the reader.
Since the story does not give the solution I cannot confirm any answer as correct, but I'm hoping there will be an answer sufficiently plausible that I'll find myself thinking Oh, of course!.