אברהם הוליד את יצחק…דבר אחר, על ידי שכתב הכתוב יצחק בן אברהם, הוזקק לומר אברהם הוליד את יצחק, לפי שהיו ליצני הדור אומרים מאבימלך נתעברה שרה, שהרי כמה שנים שהתה עם אברהם ולא נתעברה הימנו, מה עשה הקב”ה, צר קלסתר פניו של יצחק דומה לאברהם, והעידו הכל אברהם הוליד את יצחק! (רש״י בראשית כד:מב)
Since the text wrote “Yitzchak the son of Avraham,” it became necessary to state “Avraham begot Yitzchak,” for the scorners of the generation were saying “From Avimelech did Sarah conceive, since for many years she tarried with Avraham and did not conceive from him.” What did Hakadosh Baruch Hu do? He formed the features of Yitzchak’s face similar to Avraham’s, and there attested everyone “Avraham begot Yitzchak.” (Rashi on Bereishis 24:42)
Question: Why does Rashi say “leitzonei hador” (jokesters, scorners, clowns, etc.). It seems quite a natural conclusion that Avimelech was the father, as Avraham and Sarah were married for many years and did not have children, and as Rash explains, exactly nine months after Avimelech took Sarah, Sarah gave birth to Yitzchak!
The Kehillas Yitzchak explains that in truth, everyone knew that it was impossible for Avimelech to have fathered Yitzchak, for the entire world was aware of the great miracle of Asher Otzar (see Rashi on pasuk 20:9) making it impossible for Avimelech to have been the father.
However, this does not stop the laytzonim. They say what sounds good, regardless of how it jives with reality. With the passage of time, even the most sensational of miracles falls into the background. But a good piece of gossip will still be going strong many years after it is first said. Therefore, the leitzonei hador, who continued to say ludicrous things about Avraham, indeed “forced” Hakadosh Baruch Hu to make a lasting testament to the real truth.
We can also suggest a different explanation, for it is indeed a laytzonos, a foolishness, to say that Avimelech fathered Yitzchak, for a different reason. Obviously the problem with conceiving was with Sarah, and not Avraham. This can be seen quite clearly, by the fact that at the time of the episode with Avimelech, Yishmael was already 13 years old. So indeed Avraham was not the cause of the problem, and anyone saying so was indeed speaking laytzonos.
But yet again, we see that laytzonim only see things and say things as they wish, but not as common sense would demand.
How do we perceive things?