What I notice in Genesis 18 is how Sarah's faith develops. She laughs to herself when she is told as an elderly woman that she will bear a child. She is no fool. She knows that the elderly never give birth. Then she is rebuked and reminded that nothing is impossible for the LORD. Then she is embarrassed at having laughed it off and fearfully begins to take more seriously the reality of living in a world that has more going on within it than she is often aware.