How different was the “How will I know?” of Zechariah and the “How is this possible?” of Mary?
They seem so similar, but the reaction of Gabriel could not be more different.
It appears to be due to the underlying nature of each one’s question. Zechariah asks a question that only masquerades as curiosity. “How will I know?” has an obvious answer: when it happens, you nincompoop. This question is not one of curiosity, but of doubt.
Mary, on the other hand, asks a question out of sincere curiosity, as she clearly does not doubt it will happen, but merely asks how.
Gabriel reads the heart of each questioner accurately, as you would expect of a supernatural being.