Two questions from two different people.
Zacharias was an elderly priest having served the Lord for years
Mary was a young virgin living in the rather tough town of Nazareth.
Zacharias was told he and his wife would have a son in their elderly years.
Mary was told she would have a son without having relations with a man.
Zacharies doubted the angel's word.
Mary believe the angel but wondered how this could happen.
The angel disciplined Zacharias' lack of belief with a silent tounge for nine months!
The angel answered Mary's concern with an explanation.
As noted, Zacharias' question came from his unbelief. Mary's question implied she believed but wondered how to understand what was promised.
When you read God's promises do you doubt, or do you believe even if you don't understand how the promise will be fulfilled?