‘And Sarah laughed within herself, saying, “When I have grown old shall I have pleasure, my lord being old also?”.'
The words she overhears make Sarah laugh to herself. The idea is preposterous. The pleasure refers to the pleasure of birth, the joy when a child comes into the world (compare Psalms 113:9; John 16:21). Her laugh is a mark of unbelief. The promises previously given have been quite clear (Genesis 17:19; Genesis 17:21).
It is probable that Sarah is not yet aware of who the visitor is. But her expression may have been enough to give away her amusement. There is a poignancy in her words. The word for ‘grown old' means ‘worn out'. She is beyond usefulness. But with God no one is ever ‘too old' to be used.