In this parsha we learn how it says that Yitschak the son of Avraham, And Avraham the father of Yitschak but it says in the midrash when Avraham and & Sarah were going to Avi-melech Avraham told Sarah of they ask say to them that you are my sister .Avi-melech took Sarah for one night and Hashem gave him and everyone  in the kingdom Leprosy.Hashem came to Avi-Melech and said you took Sarah the wife of Avraham and that is why you got cursed,  Avi-Melech said but i did not know that Sarah is the wife of Avraham ,Hashem said you are right.After One month Sarah became pregnant and the mockers said Avraham and Sarah were married so long and now Sarah is pregnant they said no it might be Avi-Melech's kid but Hashem made it that Avraham and his son Yitschak were like twins,after a while Avraham asked Hashem that him and his son Yitschak should not look alike because when Yitschak went some were every one would stand and this is not honer for Avraham .So Hashem made Avraham the first old person.Before this people who where one hundred looked like the age of a twenty years old person