Derasha 3 on Metzora 15:06

English: “On which he sits” — I have only [learned] when he sits on it and touches it; from where [do we learn about] ten seats one atop another, and even a stone-masma? Scripture teaches: “And the one who sits on the article on which the zav sits shall be impure” — a place where the zav sits and conveys impurity, [if] the pure person sits [there], he becomes impure.