Most genealogists should know how people reproduce. But sometimes we forget that people don’t have to be married for that to happen. In some time periods and in some locations, having children outside of marriage was more common than a person might think. Two siblings of my great-great-grandfather in Thuringen, Germany, had several children before they were married (1830-1840 era). They were apparently in a long-term relationship with the fathers of their children as baptismal records for the children indicate that both sisters had their children with the same father.
Not the exact same father (one needs to be careful how one phrases things). One sister had her children with Mr. B and the other sister had her children with Mr. S. The sisters eventually married their respective fathers of their children.
Why does this matter? One reason is that sometimes the children used their mother’s surname as their last name and sometimes they used their father’s surname as their last name. That was confusing to me until I learned the marital history of their parents.