in a port [banamal], where the boats are drawn up onto shore, as there is a lot of mud there, which can stick to her and interpose between her and the water. Even though now, after the immersion, there is no mud stuck to her feet or body, one can say that perhaps while she walked back after immersing the mud fell off. Since one cannot be certain that there was no mud stuck to her body while she immersed, she may not immerse in a port.

The Gemara relates that Shmuel’s father prepared ritual baths for his daughters in the days of Nisan, by digging holes and letting them fill with natural rainwater, and he placed mats in the Euphrates River in the days of Tishrei. Since the water was shallow and the riverbed was muddy, he placed mats on the riverbed so that they could immerse without getting dirty.

The Gemara cites a similar halakha involving the interposition of an item during immersion. Rav Giddel says that Rav says: If a woman gave a cooked dish to her son, and afterward she immersed in a ritual bath and ascended from the ritual bath, the immersion is ineffective for her. The reason is that even though now, after the immersion, there is no food stuck to her body, one can say that perhaps as she walked back after immersing the food fell off. Since one cannot be certain that there was no food stuck to her body while she immersed, she should not handle food immediately before immersing.

Rami bar Abba said: With regard to these bloodletting incisions [ravdei dekhusilta], until three days have passed since the bloodletting, when they have not yet formed a hard scab, they do not interpose and invalidate an immersion. From this point forward they interpose, as the scab is too hard for the water to penetrate.

Mar Ukva said: With regard to mucus that is in the eye, if it is moist, it does not interpose and invalidate an immersion. But if the mucus is dry and hard it interposes. When is it called dry? It is called dry from the time that it begins to turn yellow.

Shmuel says: Eye shadow that is in the eye does not interpose and invalidate an immersion. But eye shadow that is on or around the eye interposes. And if the woman’s eyes were constantly blinking, then even eye shadow that is on or around the eye does not interpose, as the constant blinking removes the eye shadow.

Rabbi Yoḥanan says: If a woman opened her eyes very wide or closed her eyes very tightly while immersing, this immersion is ineffective for her. Such open or closed eyelids prevent the water from reaching the creases around the eyes.

Reish Lakish says: A woman may immerse herself in a ritual bath only in the manner that she grows, i.e., she may not force her arms to her sides or close her legs tightly. She is not obligated to spread her limbs widely, but simply stand in her normal manner. As we learned in a mishna (Nega’im 2:4): When a man has a leprous mark between his legs and stands before a priest for inspection, he should appear like one who is hoeing, i.e., with his legs slightly apart, and if it is under his arm, he should appear like one who is harvesting olives, with his arms slightly raised. If the mark is not visible when he is standing in that manner, it is not impure. By contrast, a woman with a leprous mark between her legs should appear like one who is weaving, and if the mark is beneath her breast she should appear like a woman who is nursing her son.

Rabba bar Rav Huna says: A single hair tied in a knot interposes and invalidates an immersion.