What is the source for tevilat keilim, and why do we do it?
Answer:
According to most auathorities, tevilat keilim is a Torah mitzvah, which we learn from the verse in Bamidbar (31:21-23) concerning the vessels that the Children of Israel captured from Midyan. The Jerusalem Talmud (Avodah Zarah 5:15) explains the reason for this: The utensils left the tumah of non-Jews, and entered the kedushah of Israel.