The Gemara (110b) states in the name of R. Nechunya that a number of practices are based on a halakha li’Moshe mi’Sinai: עשר נטיעות, ערבה, וניסוך המים – the allowance to work a field, up until the very start of the Shmita year, if it is of a certain area and has 10 new evenly-spaced saplings planted in it; the practice for Kohanim in the Temple to carry the arvah, the willow, around the altar on every day of Sukkot; and the practice to do water libations on the altar in the Temple on every day of Sukkot.
This is an odd grouping of practices, for while the last 2 are definitely linked – they are Temple-based practices done on Sukkot – the first one does not seem to fit the grouping at all. Rashi, in Sukkot (34a, s.v. eser nitiyot) addresses this and states:
These three were asked about at the same time in the beit midrash in regards to their origin – how do they derive from the Torah? And the response was that they are based on halakha li’Moshe mi’Sinai. The one who heard this response memorized the grouping [of these three laws] the way that he had heard it [and repeated it over in this fashion.] In a similar fashion, we find “שיעורים, חציצין ומחיצות ,” the teaching that “minimum sizes, obstructions, and laws regarding dividers [are a halakha li’Moshe mi’Sinai]”. The one who heard these [being taught] ordered them according to the way he had originially heard them [with these three grouped together and told over this teaching in a similar fashion].
According to Rashi, the grouping is not thematic, but reflects the realities of an oral tradition, that unrelated material is sometimes taught together and remembered in a similar fashion.
Following Rashi’s lead, another answer suggests itself. The 3 laws in these two cases were grouped together not by chance -based on how the question was first raised – but because such a grouping made it easier to remember them. How so? Not due to thematic similarity, but due to similarity in sound, or a mnemonic. The group שיעורין, חציצין, ומחיצות is obviously an example of this – the first two words rhyme (they both have an “een” ending) and the the second word – chatzizin – is alliterative with the third word – mechitzot. In the case in our Gemara, the grouping seems to be based on a mnemonic. The first case, eser netiyot, has two words, the first starting with an ayin the second with a nun. The next case, aravah, starts with an ayin, and the last case, nisukh hamayim, starts with a nun.
In an oral culture, it is understanding that the grouping and order of statements may sometimes be due to considerations of enabling easy memorization and retention. In fact, we often jump over the seemingly random words in the Gemara that are in parentheses and say “siman” at the end. What are these words? They are a mnemonic to remember a list of cases that the Gemara is about to quote. We can ignore this mnemonic because we have everything written out in front of us. But we should realize that not only the tannaitic material, but even the amoraic material – the Gemara itself – was part of an oral culture and not originally written down. Thus, these mnemonics were essential.
This point was not lost on the Rishonim. The first mishna in Baba Kama (2a) has a list of the four categories of nezikin: השור הבור המבעה וההבער, the ox, the pit, the tooth, and the fire. The order of this list does not seem to fit any understandable system, and Rashi (s.v. HaShor) and Tosafot (s.v. HaShor) spend a good deal of effort trying to understand the logic of the order. Rabbenu Peretz, however, gives what is clearl the best answer – this order is easy to remember because of the similarity in sounds. Shor and bor rhyme, and maveh and hever rhyme. This also helps explain the unusual words choice of maveh to represent either the tooth or a human who does damage.
There are many other examples were order and grouping are influenced by considerations of how the list can best be remembered. Truly, for an oral tradition going all the way back to Moshe, it helps to have such memory aids!