My daughter asked why is it that the tumas hayoledes for a girl baby is twice the length of that for a boy baby (2 weeks of tumah followed by 66 days of dmei taharah as opposed to 1 week of tumah and 33 days of dmei taharah for a boy)? Ramban and other Rishonim (e.g. Ralbag, Abarbanel, R' Bachye) generally take one of two approaches: 1) There is some relationship between tumas yoledes and the time it takes for cells to go from an embryonic stage to become a fetus, which is 40 days for a boy and 80 for a girl; 2) The recovery time for the birth of a girl is longer and more difficult than that for a boy. I think we would all agree that neither of these answers are scientifically accurate (see the first Ramban on the parsha, who seemed to be concerned enough with scientific accuracy to quote the doctors of his time and the philosophers on an issue that Chazal had already weighed in on) and I doubt an intelligent person would be satisfied with either approach. So how would you answer the question? The Netziv writes that the father always really wants a boy, so he draws closer to his wife faster after the birth of a boy than after the birth of a girl. (I challenge you to give that answer to your daughter, if you have one.) That may explain the difference between the 7 and 14 day wait, but does not explain the difference between the 33 vs. 66 days. Without resorting to a mystical explanation of tumas yoledes, what would you say?
Parenthetically, I saw pointed out that the question of why a yoledes bring a korban chatas (what did she do wrong?) should not even get off the ground, as the chatas of a mechusar kippurim as nothing to do with sin. See Rashi Kerisus 8b, "Mevi'im chatas v'lo al cheit elah le'echol b'kodhsim."