Abraham, after he is promised a son, finds humor in the fact that the Lord waited until he and his wife were quite old before making it possible for them to conceive! He laughs (Genesis 17:17), not because he doubts the Eternal's power (Romans 4:20 - 21), but out of joy that he will be a father when he turns 100!
Abraham's laughing is noticed by the Lord, who also finds humor in the miracle. God, no doubt in response to his friend's joy, decides to perform the rare act of naming the child himself! He declares the son's name will be Isaac, which in Hebrew means, "he laughs" or "laughter" (Genesis 17:19).
The Lord, taking the form of a man, a short time later visits Abraham in the plains of Mamre. He again confirms the promise of a child (Genesis 18:1 - 10). Sarah, hidden in a tent behind the Lord (Genesis 18:9 - 10), secretly overhears the promise. She finds what she hears both unbelievable and laughable! The Bible reveals she thinks to herself, "Now that I am old and worn out, can I still enjoy sex? And besides, my husband is old too" (verse 12, TEV)!
Sarah finds it incredibly hilarious that someone would suggest, in her old age, that she would not only be able to have and enjoy sex, but also become pregnant and carry a baby!
The Lord, who knows what Sarah is thinking, says, "Why did Sarah laugh and say, 'Can I really have a child when I am so old?' Is anything too hard for the Lord? . . ." (Genesis 18:14). Shocked and afraid that the "stranger" knew her thoughts, she lies about her laughter (Genesis 18:15). The Lord, who might have then turned and looked at Sarah, playfully and with a bit of humor corrects her by stating, "No, but you did laugh" (verse 15, HBFV)!
Sarah soon learns to have faith and fully appreciate the wonderful gift he made possible. After her son "laughter" (Isaac) is born, named, and circumcised, she joyfully exclaims, "God has made me laugh, so that all who hear will laugh with me" (Genesis 21:6, HBFV)!