Commentary for Bava Kamma 2:4

Which is unaccustomed and which is accustomed: 'Touching' implies touching it and pulling it and it does not gore. And the halacha is like Rav Yehuda in the explanation of the accustomed ox and the halacha is like Rav Meir in the explanation of a simple ox. And all the time that the ox is accustomed to gore it does not return to its unaccustomed status until that children can play with it and it does not damage by goring.

משיחזור בו שלשה ימים – when he sees bulls and they are not goring, they return to the innocuous state.

Which kind of animal is accounted harmless ( and which is an attested danger (? An attested danger is one that people have given testimony about [that it damaged] for three days. A harmless one is one that has refrained from damage for three days. This is according to Rabbi Judah.
Rabbi Meir says, An attested danger is one that people have given testimony about three times. A harmless one is one that children can touch and it will not gore.
We have several times already discussed the two types of danger: a muad which is an attested danger and a tam, which is harmless, meaning something that is not expected to cause damages. However, we have mostly discussed the different consequences of being a muad or being a tam. The owner of a muad that causes damage will pay full damages from the best of his land while the owner of a tam that damages will only pay half damages which cannot exceed the value of the damaging animal (see Mishnah 1:4). This mishnah discusses how an animal can move from the status of a tam to the status of a muad and vice versa, how an animal that is muad can revert to the status of tam.
In this mishnah we see a dispute between Rabbi Meir and Rabbi Judah, two of the most prevalent Rabbis in the Mishnah. According to Rabbi Judah in order for a tam animal to become muad people must testify against the animal on three different days. However, according to Rabbi Meir it is enough that people testify against the animal three times on one day for it to become a muad.
Similarly the Rabbis dispute how a muad would revert to being a tam. According to Rabbi Judah all it would need is three days, in which it had the opportunity to gore and yet it didn’t do so. However, according to Rabbi Meir it needs to be harmless enough for children to touch. Otherwise it remains a muad.

שלשה פעמים – and even on one day (i.e., doing damage thrice on one day), but the Halakha is not according to Rabbi [Meir] for they are not forewarned until they testify about it for three days.

ממשמשים בו – they pull at it and play with it and it doesn’t gore. And in this, the Halakha is according [to Rabbi Meir] that a forewarned bull does not return to its innocuous state until small children play with it [and it doesn’t gore].