The passuk says that Lemech married two wives, Ada and Tzila. Rashi tells us that in that generation they would marry two wives, one for the purpose of having children and the second for her beauty and for sexual relations. They would give the second wife a "kos shel ikrin" (some form of birth control) to make her sterile.
Ada was the wife for having children and Tzila was for her beauty.
My question is: two verses later we see Tzila having children. What happened to the kos shel ikrin that made her sterile? How could she have children?
The only anwers I have is that 1) it was birth control that did not work or 2) Tzila said she took the borth control but really she did not.
Anybody got anything better?