שתי אבנים

Two stones

שעשאם כירה

that were made into a kirah (by placing them next to each other and cementing them with clay)

ונטמאו

and that became tamei,

סמך לזו אבן אחת מכאן

and then one placed one stone next to one from here

ולזו אבן אחת מכאן

and one stone next to one from here (the four stones are arranged in a row, with the tamei pair in the middle and one new stone on either side of that pair),

חציה של זו טמא

half of this one is tamei

וחציה טהורה

and half (the half that would support a pot with the newly placed stone) is tahor,

וחציה של זו טמא

and half of this one is tamei

וחציה טהורה

and half it tahor (in other words, the halves of the tamei stones that now serve to support pots with the newly placed stones are tahor);

ניטלו טהורות

If the tahor ones are [subsequently] removed,

חזרו אלו לטמאתן

they return to their status of tumah (i.e. the two originally tamei stones become completely tamei again.

פרק ז משנה א

הקלתות

The stove bases (earthenware slabs that formed the bottom of kirahs)

of householders

that was missing a part (creating an indentation in the floor of the oven),

if [the depression is] less than three handbreadths [deep],

it [the stove] is tamei (i.e. is susceptible to tumah),

as one heats it from below

and the pot cooks above (i.e. even though the fire is now lower down, the heat still reaches to the pot above),

more than that is tahor (as the greater distance of the flame from the pot keeps it from cooking properly, it is no longer susceptible to tumah);

if one place a stone or a pebble (in such a depression of greater than three handbreadths, thus raiding the flame)

it is tahor (as these are not physically attached to the stove or its base, it is still considered 'broken'),

if he covered them with clay (if one attached the stone or pebble filling the depression with clay)

it [the stove] is susceptible to tumah

from that point on.

This was the response of Rabbi Yehuda

about a tanur

that was placed over the mouth of a pit

or the mouth of a cistern (I.e. his position in 5:6 that an tanur placed over a cistern or pit in which there is a fire is tamei only if the fire heats the tanur.)