Commentary for Chullin 174:22

כשירין איצטריכא ליה

But even in the case of rain water since it was collected [in a vessel] and poured [into the blood] it was surely intended for the purpose! - It was necessary only in the case where they were mixed without human effort.<span class="x" onmousemove="('comment',' Sc. the blood and the rain water. In this case, therefore, as the rain water by itself cannot render susceptible to uncleanness, only if the mixture has the colour of blood will it render susceptible to uncleanness.');">¹³</span>

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