Mikvaot, Chapter One, Mishnah Five
Today’s mishnah is the last mishnah that deals with the water found in pools that are less than forty seahs. The mishnah refers to a pool of water that had been made unclean and now rain has started to flow back into it. The question is at what point the pool becomes clean again.
When do they become clean?
1) Bet Shammai say: when their contents have been increased [by more than the original quantity] and they overflow.
2) Bet Hillel say: when their contents have been increased [by more than their original quantity] even if they do not overflow.
3) Rabbi Shimon says: when they overflow although their contents have not been so increased.
4) [These] are valid [for preparing dough] for hallah and for the washing of the hands.
Section one: Bet Shammai rules most strictly. In order for the pool to become clean again, the new water that has come in from the rain must be more than the original water that remained and the contents must also overflow.
Section two: Bet Hillel says that the pool need not overflow.
Section three: Rabbi Shimon rules even more leniently. If the pool was almost full and then it rained and it overflowed, the pool is clean even if there is only a small amount of rainwater.
Section four: The water in the pool is now completely clean. The mishnah emphasizes how pure the water is by stating that one could even use it to prepare hallah or two wash one’s hands. The same is true for preparing terumah. In other words, the water is not at all impure.