Can You Keep A Secret?

There is a dispute between Rav and Shmuel as to whether or not one is permitted to precisely measure flour on yom tov for the purpose of baking. Shmuel is of the opinion that flour may not be measured. This is in contradistinction to spices, which may be measured precisely on yom tov. The Mishnah Berurah explains that even a small deviation from the required amount of spices can ruin a dish. However, generally the amount of flour in a recipe can be a little off and the baked good will still be just fine. The Gemara notes that while Shmuel himself ruled it was forbidden, in his own collection of rulings from Tannaim there is a Tanna who ruled it was permitted.
Did Shmuel intend to argue on a Tanna? Abaye says that he did not. Shmuel meant to teach the practical halachah. Rashi explains that it is true that according to the letter of the law flour may be measured precisely on yom tov; however, “We don’t teach this law.” This is referred to throughout the Talmud and halachic literature as “halachah v’ein morin kein.” Regardless of whether or not this law was taught in the past, one certainly cannot measure flour precisely nowadays on yom tov. Tosfos explains that the reason the Talmud offered for the lenient opinion no longer applies. Hence, all agree that today we do not measure flour precisely on yom tov.