After God changes Abram and Sarai's names to Abraham and Sarah, he tells Abraham that he will bear a second son by Sarah named Isaac, with whom a new covenant would be established. In response, Abraham began to laugh, as both he and Sarah were well beyond natural child-bearing age. Some time later, three men who Abraham identifies as messengers of God visit him and Sarah, and Abraham treats them to food and niceties. They repeat the prophecy that Sarah would bear a child, promising Isaac's birth within a year's time, at which point Sarah laughs in disbelief. God questions why the pair laughed in disbelief at his words, and if it is because they believe such things were not within his power. Now afraid, they futilely deny ever having laughed at God's words.
Time passes as Isaac is born.
–Wikipedia