אור החיים — ותצחק שרה בקרבה

Q — Avraham laughed at the news of Yitzchak's birth (ויצחק) and God did not object — so why did God rebuke Sarah for the very same laughter? Rashi distinguishes Avraham's joyful laughter from Sarah's mocking laughter, but Or HaChaim rejects relying on two opposite meanings of one root (צח"ק).

A — Or HaChaim answers from the timing, not the type, of the laughter. Sarah's laughter, too, was joyful and faith-filled — but it came only after she saw physical changes in her body ('אחרי בלותי היתה לי עדנה'), not at the moment of the annunciation. That reveals she did not believe the promise on its own word, but only once she saw the proof. Avraham, by contrast, rejoiced immediately upon hearing. God objected to her needing the evidence first.