If there is a Chatzitza present on most of the hair, the Tevillah is passul. Rashi describes the hair being knotted one by one. Why is this considered a Chatitza on most of the hair if only a minute portion of each hair contains the knot (which prevents water from entering the interior of that knot)?
We learn that Shiurim were learned from Sinai. We also learn about the seeming uncertainty as to the exact size of an Amma. How can there be a safek about something which was taught through a Halacha lemoshe misinai (see Ramba”m’s Introduction to Mishnah)?