Shavua Tov. Today׳s Parsha discusses the war with the Midyanites. The interesting aspect of this battle was that an important Halacha was learned that we continue to observe today. It concerned the spoils of war that were taken by the Jewish people.
The people came to Moshe and asked what they were to do with the pots and pans and cooking utensils that were once used by the Midyanites. They were taught both the rules of הגעלת כלים, purging utensils in fire, and the rules of טבילת כלים, dipping such utensils made or used by a non-Jew in a Mikva or natural body of water.
The Rabbis explained that fire has a special characteristic. In Talmudic terms, it say that אש משאב שאיב, fire is able to draw out that which is absorbed in the walls of the utensils.
The case of טבילת כלים is considered an אסמכתא, that which is learned indirectly by way of a story. When one is asked how we know that we must dip utensils in a Mikva, the answer is כלי מדין. An אסמכתא is still considered prohibited by the Rabbis, but is the strongest rabbinical law since it is based on a story in the Torah.