The biological mystery of unlike offspring from the same parents is the challenge of parenting some children.
Some Torah portions lend themselves very easily to sermons. Yitro, which contains the giving of the Ten Commandments has lots of material about which to talk. Others are more challenging, like Tazria-Metzorah, which has extensive discussions about skin diseases, inflammations and rashes.
This week's Torah portion begins with an issue that is a recurrent one for our foremothers -- difficulty conceiving. As Sarah before her and Rachel after her, Rebecca has trouble getting pregnant. After her husband Isaac pleads with God, she does conceive. But the pregnancy is a painful one -- so much so that Rebecca cries out with words to the effect of, "Would that I did not exist!" Out of this depth of despair she approaches God.