Kelim, Chapter Six, Mishnah Four
1) If two stones were made into a stove and they became defiled, and a stone was set up near the outer side of the one and another stone near the outer side of the other, [the inner half] of each [inner stones] remains unclean while [the outer] half of each [of these stones] is clean.
2) If the clean stones are removed the others revert to their impurity.
Section one: At the outset there are simply two stones attached to the ground and made into a stove. These stones become defiled. Then he put another stone on each side, creating three stoves made of four stones. The inner half of each of the original stones remains impure because it serves the inside stove. The outer half which faces the pure stone becomes pure because it has now been made into a new stove.
Section two: If he then removes the outer stones, the inner stones revert to their earlier state of impuritywhich means they are completely impure.