Question: What's the truth about Mayim Achronim? I've noticed that some people only put a few drops of water on their fingers, but what amount is needed? Is it necessary to perform Mayin Achronim after eating a meal during the week?
Answer: There are two primary Gemaras about Mayim Achronim, each giving different reasons for the practice. Based on two verses in Vayikra, the Gemara in Berachos (53b) states that we wash Mayim Achronim for holiness. Reciting Hashem's name in Birchas Hamazon with dirty hands is considered inappropriate. The Gemara in Eruvin (17b) explains that a type of salt called Melach Sedomis can cause blindness if it comes into contact with a person's eyes. The concern is that remnants of this salt may remain on one's hands after a meal, and washing removes this risk.
The Ritva (Eruvin 17b) suggests that these two sources teach two distinct aspects of Mayim Achronim. The Gemara in Berachos emphasizes a mitzvah d'Rabbanan for enhanced holiness, while the Gemara in Eruvin brings an obligation to prevent harm from Melach Sedomis. The Shulchan Aruch (181:1) rules based on these Gemaras that Mayim Achronim Chova—Mayim Achronim is an obligation.
Regarding the water, there are disputes among the Poskim on two points: how much of the hand must be washed and how much water is required. The Shulchan Aruch (181:4) rules like the Rashba (Bayis 6, Shaar 1, Ch. 9) that one only needs to wash until the second knuckle. This is also the view of the Chayei Adam (46:1) and Aruch HaShulchan (181:7). However, the Biur Halacha (s.v. ad) cites the opinion of the GRA, who holds that the entire hand should be washed. The Biur Halacha concludes that following the GRA's stricter view is good.
The Bais Yosef (181:8) writes that Mayim Achronim does not have a set amount of water. Mishna Berura (181:19) quotes this view and explains that even a tiny amount is sufficient if it can clean the areas up to the second knuckle. The Mishnah Berurah also brings the opinion of the Vilna Gaon, who requires a revi'is of water. The Elya Rabba (181:3) and the Aruch HaShulchan (181:8) rule that any amount of water is acceptable.
Although the Shulchan Aruch (181:1) rules that Mayim Achronim Chova, it also cites the opinion of Tosafos (181:10) that nowadays there is no obligation to wash Mayim Achronim (because the danger of Melach Sedomis no longer applies), however, Tosafos adds that if someone is fastidious (אסטניס) and regularly washes their hands after meals, they must still observe Mayim Achronim. This is also the opinion of the Rosh (Berachos 8:6).
Some mistakenly believe that Mayim Achronim only applies to Shabbos, perhaps because people use special Mayim Achronim sets for Shabbos meals, making it more noticeable. However, the Poskim make no distinction between Shabbos and weekdays; the mitzvah applies equally to both.
In short, there are two reasons for Mayim Achronim: extra holiness and removing the risk of Melach Sedomis. The Shulchan Aruch rules that it is obligatory but also cites the lenient view of Tosafos that it is no longer required in modern times. While even a small amount of water suffices for washing up to the second knuckle, washing the entire hand using a revi'is of water is preferable. There is no difference between Shabbos and weekdays regarding the mitzvah.
