Génesis 18:12 Comentario: Rashi, Rashbam, Radak & Sforno

בקרבה [SARAH LAUGHED] WITHIN HERSELF (or, REGARDING HER INSIDE) — She reflected on her physical condition, saying, “Is it possible that this womb shall bear a child, that these dried-up breasts shall give forth milk” (Midrash Tanchuma, Shoftim 18).

עדנה, a form of the skin becoming elastic and the wrinkles straightening out.

ותצחק שרה בקרבה, Sarah laughed derisively internally, not audibly; she did not believe that the man who had made the prediction was an angel, though she assumed that he was a prophet.

עדנה means glistening flesh (skin), and so it is used in the Mishna (Menachot 86a): it makes the hair fall out and the skin smooth (מעדן).