Kelim, Chapter Nine, Mishnah Two
1) A jar that was full of clean liquids, with a siphon in it, and it had a tightly fitting cover and was in a tent in which there was a corpse:
a) Bet Shammai says: both the jar and the liquids are clean but the siphon is unclean.
b) And Bet Hillel says: the siphon also is clean.
2) Bet Hillel changed their mind and ruled in agreement with Bet Shammai.
Section one: The siphon is made of metal, and as we learned, an earthenware vessel with a tightly fitting cover does not prevent impurity from defiling metal vessels that are within it. Therefore, Bet Shammai rules that the metal siphon is impure. The jar and the liquids inside are clean because the covered earthenware vessel does protect them from the impurity.
Section two: At first Bet Hillel disagreed and held that even the siphon was pure. However, they eventually changed their mind and agreed with Bet Shammai. This is a phenomenon that occurs occasionally in the Mishnah. Bet Hillel at first disagrees with Bet Shammai, but eventually they change their mind.