Genesis 18:11-12

"And Abraham and Sarah were old, and well stricken in age; it had ceased to be with Sarah after the manner of women. And Sarah laughed within herself, saying, After I am waxed old shall I have pleasure, my lord being old also?"

From the human standpoint, it was impossible for a woman long after the onset of menopause to give birth to a child, and Sarah's derisive laughter seemed to her to be the most appropriate response to what the stranger had said. As yet, it was not fully evident to her that God Himself was behind the promise. To her credit, in time, she believed and was empowered by The Highest to conceive and bear Isaac, appropriately enough called "laughter." Willis noted that, "The Hebrew word [~titschaq] is very similar to the word Isaac, [~Yitschaq]";[16] and that is why the meaning of the name Isaac is usually given as "laughter." He was, by his very name, a perpetual reminder to both Abraham and Sarah that, in a genuine sense, he was a supernatural gift from God, called forever afterward, "the son of promise."