Parshat Lech L'Cha
In 17:17 after Hashem tells Avraham that he will be having a child, Avraham falls on his face in laughter saying in his heart can a hundred year old man have a child born? And Sarah that is 90 years old can bear a child?
What's the big deal? He just had a child a few years ago at the young age of 86? That could happen but not by 100? 86 is normal and 100 is crazy?
Even according to Rashi who compares Avraham to previous generations that had children at 130+, Rashi still says that that had been well before Avraham's time. But in Avraham's time it was normal to have children at 60 or 70. So even according to Rashi 86 was still well out of the normal range of child bearing, so why was 100 so unusual that it caused Avraham to break out in laughter while talking with Hashem?