Zavim, Chapter 1, Mishnah 4. The opening chapter of this masechta counts re'iyos: a man who experiences one flow, two flows, or three flows stands at three different levels of tumah, and three re'iyos make him a zav gamur, obligated in a korban. That raises a practical question that this mishnah comes to answer: when do we count a flow as one re'iyah, and when does a break in the middle turn it into two?
The measure of an interruption
How large must the break be? The mishnah fixes it with the phrase "kedei tevilah vesipug", meaning the stretch of time in which someone could go down into a mikveh, immerse, and afterward towel himself dry. The perek already gave this quantity a concrete measure: the time consumed by a walk of fifty amos, something in the range of fifty meters. Once a gap of that size opens up, it counts as a real hefsek, severing whatever preceded it from whatever comes next.
The cases
The first case: "Ra'ah achas", a single re'iyah occurred, "vehefsik kedei tevilah vesipug", and it was followed by a gap of precisely that measure.
"Va'achar kach ra'ah shtayim, o achas merubah kishtayim": and afterward he experienced two re'iyos, or a single extended flow whose length equals two re'iyos, so that it is counted as two.
Or the same components in the opposite order: "O ra'ah shtayim, o achas merubah kishtayim, vehefsik kedei tevilah vesipug, va'achar kach ra'ah achas": he experienced two re'iyos, or one prolonged flow counted as two, then paused for the span of immersing and drying, and after that had a single re'iyah.
The ruling
"Harei zeh zav gamur": in all of these cases he is a full zav, because we have before us three re'iyos. The break of kedei tevilah vesipug does its work in both directions: it divides the earlier flow from the later one, and a flow long enough to equal two re'iyos is credited as two.
When the gap is too short
If, however, the interval between the flows was shorter than the time needed to immerse and dry off, it is not a hefsek at all. In that case the two experiences are not treated as separate events; they are counted together as a single re'iyah.
This is where the fourth mishnah leaves us: the tally of re'iyos is not decided by how many times a flow appears, but by the measured pauses between them, and by the length of a flow that stretches on as though it were two.