What does the Torah mean when it writes that "also the sons of Machir ben Menasheh were born on the knees of Yosef"?
Rashi and Targum Onkelos: The Torah means that they were raised on his knees.
Targum Yonasan: It means that he circumcised them. 1
Rashbam: Bnei Machir means grandsons (the fourth generation after Yosef). 2
Moshav Zekenim: Even though Yakov did not bless Menasheh like (i.e. as much as) he blessed Efrayim, Hashem blessed Menasheh, and Yosef saw grandsons also from Menasheh.
Or that he was the Sandek at the B'ris (See Peirush Na'ar Yonasan).
What is the Chidush that Yosef saw a fourth generation? He lived to 110 years old. Even a man of 90 can see this!
Moshav Zekenim: It was a Chidush that all of them were alive. Not one of them had died.
Hadar Zekenim: He saw a fourth generation counting from Efrayim. 1
This is a Chidush, for Yosef did not marry until he was 30, and initially there were very few girls for his sons to marry. Presumably, he would want them to marry descendants of Yakov, and not Mitzriyos. Among Yordei Mitzrayim were 67 males, and only two single females who might have married Yosef's sons, Serach and Dinah (and Dinah was much older than them, and some say that she married Shimon or Iyov)! Refer to 46:26:1:2. (PF)