rabbi twisted’s favorite, loosely translated from Sforno Breshis18;12
“and Sarah laughed” because she thought the statement of the angel was only a blessing from a prophet, and as in the case of Elisha, and not a true prophecy or directive of the Holy One blessed be He. For what is impossible for the aged is not achieved through the blessing of any prophet, indeed the return of youth after old age is alike to the revival of the dead, which can only happen through a directive of the Holy One, or through a prayer which merits grace (chen) from Him.