WHY DID SARAH LAUGH?

"Ohr HaChayim notices that Avraham also laughed when Hashem promised him that he would have a son with Sarah (ibid. 17:17). What is the difference between Avraham’s and Sarah's laughter? Onkelos translates Avraham's laughter as “chadei” - to rejoice - and Sarah's laughter “chayechet” - to mock. How can the same word be translated into two completely different meanings? Mattis Weinberg explains that Avraham spoke in a general language saying: “…Shall a child be born to him that is a hundred years old?” whereas Sarah placed herself in the center: “…after I was faded, I had pleasure?” Avraham spoke in the future tense indicating that the future might deviate from the usual way of the past. However, Sarah spoke in the past tense emphasizing that what was will continue to be, disbelieving the thought that the nature of her body would be altered. Nonetheless, Kli Yakar ascribes it to her praise that she only laughed within herself, never daring to bring this laughter to her lips. Sarah was only reproved for the little laughter in her thoughts. This only emphasizes her holy level; the sin Sarah had to refine was so subtle, so small.