And these are the generations of Isaac son of Abraham. We relate Isaac to Abraham to fulfill what is stated, "And the glory of children is their parents." (Proverbs 17:6) Abraham sired Isaac. We relate Abraham to Isaac to fulfill what is stated, "Grandchildren are the crown of their elders", and all the more so children. Another interpretation: Abraham sired Isaac. Because people were saying that Isaac was the son of Avimelech, therefore the Holy One Blessed Be He made Isaac's face similar to Abraham's, in order that all would know that Isaac is the son of Abraham and not the son of Avimelech. Therefore it is stated, "Abraham sired Isaac." And all who see Isaac would say that he is certainly Abraham's son, since Abraham sired Isaac.