Then Sarah denied, saying, I laughed not; for she was afraid. And he said,
Nay; but you did laugh.
Read Chapter 18
George Leo Haydock
Afraid; which does not entirely clear her of sin: for though she might innocently laugh, if she thought the person who spoke was only a man, yet she ought not to have told an untruth; and if she reflected, that he had disclosed what she supposed no one knew, and thereby manifested his superiority over man, her denial was still more inexcusable. But she was taken, as it were, by surprise; and therefore the Lord reproves her very gently. (Haydock)