Points you don’t want to miss:

In identifying precisely what leftover presence of urine would restrict Torah study and blessings, the Talmud discusses different standards to consider. These include, after absorption into a cloth or the ground, if 1) it is wet enough that if touched by hand, it would make the hand wet enough to make something else wet, 2) it is merely wet, or 3) it is absorbed but its imprint/stain is still seen.

These precise standards come up in other areas of Jewish law. For example in the laws of Shabbos (when squeezing a cloth is prohibited), one may not handle a cloth that is so wet that if touched by hand would make the hand wet enough to wet something else. But if the cloth is not that wet (such as is typical after the washing machine has finished the final spin cycle and largely wrung the clothing out of their moisture) it may be handled on Shabbos, and is not Muktzeh.

Another related topic is the law of “Kisuy Hadam,” the Mitzvah to cover the blood of a bird (or non-domesticated animal, such as a deer) after ritual slaughter. If per-chance someone did not cover the blood promptly, this Mitzva is still relevant as long as the impression/stain of the blood is still present on the dirt. (see Shulchan Aruch-Yoreh Deah 28:16)

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Praying the Amidah is considered standing “In front of the King.” As such, the level of reverence required is greater that what is required to recite Shema. According to strict law, one could recite Shema without a shirt. But for prayer, as it is considered standing “In front of the King,” a shirt is required.

On a practical level, it is best to be properly covered  for Shema as well. But the Talmud always pushes to identify what is absolutely required.  In this way we know what leniencies may be used in unusual cases such as one who realizes that he needs to saw Shema now, or he will miss the proper time, and was on his way into a shower, and is not wearing a shirt at this moment. Knowing that a shirt is not technically required enables him to recite Shema and do the Mitzva immediately, without delay.

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