“These are the generations of Yitzchak the son of Avraham, Avraham fathered Yitzchak.” (Bereishit 25:19)
Readers of this first verse cannot help but notice the redundancy between the first and second clause of the same sentence. If Yitzchak was the son of Avraham, then indeed Avraham fathered Yitzchak. Rashi, ever sensitive to the subtleties of language tells us of a backstory, which explains as to why this clarification was in order.
Yitzchak’s miraculous birth to a ninety year old mother and hundred year old father, was fodder for the ancestors of the stand-up comics on late night television. Rashi writes: “the comics of the generation asserted that Sara became pregnant from Avimelech…”(Rashi Bereishit 25:19)
Whether these people should be called “comics” or “cynics” or “scorners”, is unclear, what is clear is that they went for the cheap laugh at the expense of Yitzchak – and Avraham and Sarah. They did not wish to entertain the possibility of a family that lived a life of holiness, and were blessed with a miracle. Rather dirty minds with wonderful imaginations provided a more rational explanation for Yitzchak’s birth, with Avraham and Sarah – suffering the collateral damage of these poisonous jibes.
For some reason they considered the pregnancy of a ninety year old woman who has suffered decades of infertility, to be rationale, and the virility of an elderly man – who had recently fathered a child – to be irrational. But such is the power of cynicism, to plant seeds of doubt, even where the alternative narrative is equally or more implausible.
The identity of these comics is not shared, yet there is a earlier reference which may provide the hints to identify one of them. When Yitzchak was born someone laughed – or perhaps smirked, this laughter brought the ire or Sarah:
And Sarah saw the son of Hagar the Egyptian, whom she had borne to Abraham, smirking. And Sarah said to Abraham, "Drive out this handmaid and her son, for the son of this handmaid shall not inherit with my son, with Yitzchak." (Beresihit 21:9,10)
Sarah’s reaction seems strange the entire existence of Yitzchak is one surrounding by joy and laughter. Avraham, laughed when he was informed that Sarah would have a child, Sarah laughed. Sarah even said after giving birth, that all who hear of her happy news will laugh out of joy. Why then is Sarah incensed at the laughter of Yishmael? The answer is he wasn’t laughing – he was smirking. A number of commentaries connect the smirking of Yishmael with the stand-up comics. He was laughing about the possibility of Avraham fathering Yitzchak.
At first glance this is bizarre – for if Avraham could not have a child – what is then the real identity and lineage of Yishmael? Apparently Yishmael was troubled by the appearance of a new son of Avraham – one who would now challenge the birthright of Yishmael – this is in fact the precise message that Sarah responded with:
And Sarah said to Abraham, "Drive out this handmaid and her son, for the son of this handmaid shall not inherit with my son, with Yitzchak." (Bereishit 21:10)
Sarah does not say that Yishmael will be a bad influence, or give other reasons for his expulsion – rather the focal point is inheritance.
We are unfortunately familiar with suicide bombers whose blind hatred of others is so profound they harm themselves in the process. Here we find a suicide comic, who is willing to harm his own chances of inheritance as long as he harms the object of his jealousy and hatred in the process.
Indeed Avraham fathered Yitzchak, and Yitzchak was the son of Avraham, a miraculous child, given to wonderful parents. Some people were overjoyed with the miracle. For others it fueled laughter of a different type, smirking, knowing glances and cynicism. With all the progress of the past four thousand years, the world has not changed all that much.
For a more in-depth analysis see: http://arikahn.blogspot.co.il/2013/10/audio-and-essays-parashat-toldot.html
 Seforno 21:9, Beit Yitzchak 21:9, Sefer Maaseh Hashem Chelek Maase Avot chapter 19