Makhshirin, Chapter Five, Mishnah One
1) One who immersed himself in a river and then there was in front of him another river and he crossed it, the second [water] purifies the first [water].
2) If his fellow pushed him in during exercise or his beast [pushed him in], the second [water] purifies the first [water].
3) But if [he did it] out of playfulness, it comes under the law of if water be put.
Section one: When he gets out of the first river, the water that remains on his body causes produce to be susceptible to impurity, because he intentionally went into the river to bathe. Then he has to cross another river but he doesn’t need this river to bathe. The water from the second river does not cause susceptibility because he didn’t want to get wet, he just wanted to cross. The water on him from the second river therefore purifies, or causes not to make susceptible, the water from the first river.
Section two: A person comes up out of the water after bathing. Then he gets pushed in again, either because he and his friend were exercising or his animal pushes him in. The second time in the river is not something he wanted, so this water “purifies” the water that was on him from the first time. Note that here he goes into the same river twicethe water from the first time does cause susceptibility but the water from the second time does not.
However, if he and his friend were just playing around, then entering the water is something that they wanted. Therefore, this water too causes susceptibility. Again, it is all a matter of intent.