Zavim, Chapter 1, Mishnah 5. The whole first chapter of Zavim is occupied with counting. How many re'iyos (flows) did this man have, and were they spread over one day, two days or three? The count is what determines everything: whether he is a lesser zav, tamei but nothing more, or a zav gadol who must undergo the fuller purification and bring a korban. Our mishnah introduces a remarkable principle into that counting: sometimes a single flow, if it goes on long enough, is counted as more than one.
One re'iyah with the measure of three
The mishnah begins: "Ra'ah achas meruba k'shalosh" - there was only one flow, yet it continued for as long as three ordinary re'iyos would take. And how do we know what that duration is? The Tanna supplies a familiar yardstick: "k'shehi mi'Gadyon l'Shiluach" - the stretch of time a person spends traveling between Gadyon and the Shiluach, that famous spring of Yerushalayim.
The mishnah then offers a second way of expressing the same span: "shehein kdei shtei tevilos u'shnei sipugin" - the time needed for two immersions in a mikveh together with two dryings off afterward. Both descriptions come out to roughly the same amount of time, which is approximately the time it takes a person to walk one hundred amos.
Of such a man the mishnah rules: "harei zeh zav gamur" - he is a complete zav. His single flow was so drawn out that halachah treats it as three re'iyos, and he therefore carries the full weight of the status: the more elaborate process of taharah, culminating in the bringing of a korban.
One re'iyah with the measure of two
The mishnah continues with a shorter case: "ra'ah achas meruba kishtayim" - the flow was prolonged, but only to the extent of two re'iyos, that is, the time of one immersion and one drying off. Here he does not reach the level of a zav gadol, yet he is not left untouched either. He is metamei mishkav u'moshav: whatever he lies upon or sits upon becomes tamei through him, precisely because that one stretched out re'iyah is counted as two.
How, then, does such a man become tahor again? The mishnah spells it out: he is "tzarich bi'as mayim chayim", he must enter living, flowing waters, and yet he is "patur min hakorban", exempt from any offering. A korban is the obligation of the zav gadol alone, and that title demands a tally of three re'iyos; this man's account stands at two.
The view of Rebbi Yose
The mishnah ends with an objection from Rebbi Yose. In his view, "lo amru achas meruba", the halachah of a stretched out single flow being multiplied, applies "ela im kein yesh bah kdei shalosh": only where the flow lasted the full measure of three, that hundred amah walk. Counting one re'iyah as though it were two, however, is something Rebbi Yose maintains was never taught at all. The disagreement rests on how each Tanna understands the pesukim in Sefer Vayikra: the first opinion finds in them a source for one flow being credited as two, and Rebbi Yose does not.
So a single, unbroken flow may be worth one, two or three re'iyos, depending entirely on how long it lasted, and according to Rebbi Yose only the full measure of three carries that power. The clock, in this halachah, does the counting.