Parah, Chapter 8, Mishnah 6. As this chapter moves beyond the laws of the mei chatas, it takes up general principles of tumah and taharah, and especially the special severity that the Sages attached to liquids that have become tamei. Our Mishnah applies that principle to earthenware.
Listen to the wording: "Ein k'li cheres", an earthenware utensil, "metamei chavero", does not impart tumah to its fellow utensil, "ela mashkeh", that job belongs to a liquid alone. And the Mishnah goes on: "Nitma mashkeh", once the liquid has itself contracted tumah, "timahu", it makes that second utensil tamei.
Two established rules combine to produce this halachah. Utensils take on tumah only from an av hatumah, tumah in its highest grade, standing at the head of the chain. Earthenware, however much tumah it may have absorbed, is never able to climb to that grade. The result is that a tamei earthenware utensil has no direct means of passing its tumah on to a second utensil. One avenue alone remains open to it, and that avenue is a liquid.
Accordingly, once a liquid touched the tamei earthenware and picked up tumah from it, that liquid is now able to impart tumah to another utensil. Once more we are dealing with a rabbinic stringency, and the model for it is the discharge of a zav. Such a discharge carries the rank of an av hatumah, and on the level of Torah law it makes utensils tamei. Working from that pattern, the Sages gave tamei liquid a comparable power over utensils.
The Mishnah captures how strange this chain is by putting words into the mouth of the second utensil: "Harei zeh omer", this one declares, "metamecha lo tim'uni", whatever gave you your tumah had no power to give me mine, "ve'atah timtani", and yet you have gone and made me tamei. The earthenware, barred from ever holding the rank of an av hatumah, could not have altered this utensil's status in the slightest. The liquid, whose tumah came from nowhere else but that earthenware, is able to do exactly that, and does.
The picture that emerges is precise: earthenware cannot pass its tumah to another vessel on its own, but a liquid that has become tamei from it becomes the messenger that carries the tumah further than its own source ever could.