Who the mother of one’s child is matters in more ways that one can ever rationally know. God doesn’t tell Avraham why Sarah must be the mother of his heir, but we readers begin to suspect that she holds the key to the future. Avraham seems to have learned this by the time he sends his servant back to Terah’s tribe to seek a wife for Yitzhak. The servant specifies a test for the bride-to-be: that she not only offer him a drink from her pitcher, but also care for his thirsty camels, at considerable investment of effort. In other words, he devises a test for kindness—kindness to the stranger and to beasts of burden.