What is a kind with its own kind? Wheat does not combine with anything except with spelt. Barley combines with everything, except for wheat. R' Yochanan ben Nuri says: The other species combine with one another.

Two [doughs] of a kav and a rice [dough] of a kav or a terumah [dough] of a kav in the middle do not combine. [If] something whose challah was taken [was] in the middle, they combine, since it was once obligated in challah.

A kav of new [flour] and a kav of old [flour] that “bit” into one another, R' Yishmael says: He takes [challah] from the middle. But the Sages forbid [this]. [If] someone took challah from a kav [of dough], R' Akiva says: [It is] challah. But the Sages say: It is not challah.

Two kav [doughs]; the challah of this one was taken by itself, and [the challah] of this one by itself, [and] he then made them into one dough, R' Akiva exempts [the dough], but the Sages obligate [it]. It thus emerges that his stringency is his leniency.