Bava Kamma, Chapter 2, Mishnah 4. The Mishnah continues analyzing the five ways an animal becomes a mu'ad, as listed in the last mishnah of the first chapter. The third in that list was the shor hamu'ad - the ox that has established a presumption of being dangerous.
The source in the verses:
The source for this is in the Book of Exodus, Chapter 21, Verse 36: "O noda ki shor nagach hu mitmol shilshom velo yishmerennu be'alav, shalem yeshalem shor tachas hashor". That is, if it became known to the owner and it became clear to him that his ox gored with its horns, and at three different times:
"Ki shor nagach hu" - today.
"Mitmol" - yesterday.
"Shilshom" - the day before that.
Here we have three times that the ox gored. "Velo yishmerennu be'alav" - and even so, the owner did not guard his ox and did not prevent it from causing further damage, and the ox did this a fourth time - "Shalem yeshalem shor tachas hashor", and he must pay full damages for the value of the ox that gored and caused damage.
The Mishnah's question:
The Mishnah opens and asks: "Eizehu tam ve'eizehu mu'ad". All animals start with the status of a tam, and the question is twofold: when does an animal revert to being a tam, and when does it become a mu'ad in the first place? The Tanna'im disagreed on this question.
The opinion of Rabbi Yehudah:
"Mu'ad kol shehe'idu bo sheloshah yamim" - an animal is considered a mu'ad if the owner received an official warning in court on three separate occasions, that the animal is causing a certain type of damage or threatening in some way. The warning must be on three different days, as per the wording of the verse, and three times in one day are not enough: we must see a continuous pattern of unusual damage and unusual behavior from the animal. There is no need for three consecutive days, but rather three separate and distinct days.
"Vetam misheyachzeru bo sheloshah yamim" - it reverts to being a tam, and the owner is again liable for only half damages, if for another three days afterward - and again, not necessarily consecutive days, but three days without contradictory behavior between them - the animal behaved properly with other cows and did not gore, push, or bite them.
"Divrei Rabbi Yehudah". Thus, in his opinion, three separate days are required to become a mu'ad, and three separate days to cease being a mu'ad and establish a new pattern of tam status.
The opinion of Rabbi Meir:
"Rabbi Meir omer, mu'ad shehe'idu bo shalosh pe'amim" - even if the animal caused damage three times in one day, and the owner was officially warned three times in court. Even though everything happened in one day, says Rabbi Meir, all the more so we see that this animal is truly dangerous, and therefore it is promoted to the status of a mu'ad.
"Kol shehatinokos memashmashin bo ve'eino nogeach" - and its return to the status of a tam is when small children play with the animal and bother it, and it does not attack them back.
The choice of the example specifically with children is precise: the animal is not afraid of them due to their small size, and nevertheless, when they provoke it as children do and it does not react, it becomes clear that its nature is now more docile and it returns to the status of a tam.
Two ways to understand Rabbi Meir:
One way: Reverting from the status of a mu'ad to its original state is quite difficult, and it is only achieved through this test of the children bothering the animal without it reacting to them.
A second way: Rabbi Meir agrees with Rabbi Yehudah that if three separate days pass as proof that the animal is no longer biting or kicking as it used to, it reverts to being a tam. However, he adds that even one time, when we see children bothering it in their play and it does them no harm, we understand that the animal's nature is good after all.
The halachah:
The halachah follows Rabbi Yehudah, not Rabbi Meir. Therefore, a pattern of behavior over three separate days is required for the ox to become a mu'ad, and a new pattern of behavior over three separate days is needed to revert it from a mu'ad back to the status of a tam.
In summary: In this Mishnah, we learned the parameters of a mu'ad and a tam from the verse, "Or if it be known that the ox was a gorer from yesterday and the day before," from which we derive three gorings. Rabbi Yehudah requires warning over three separate days for it to become a mu'ad, and three separate days of proper behavior for it to revert to a tam. Rabbi Meir suffices with three incidents even on a single day, and establishes the reversion to a tam through the test of "hatinokos memashmeshin bo v'eino nogeach" - the children stroke it and it does not gore. There are two ways to understand his ruling. The halachah is decided in accordance with Rabbi Yehudah.