Aerith has the memorization- and arithmetical skills of a genius.
This is barely even a headcanon since it's right there in our faces in Crisis Core.
See Aerith's reaction after Zack builds a flower wagon.
In the span of 2-3 seconds Aerith is able to come up with- and count 23 wishes. Let's think about that for a second. How many wishes, critiques, comments etc can you come up with even for the most thought-provoking of subjects in less than thirty seconds? For that matter, can you easily count them all in just a few seconds? Chances are the answer to the latter question is "no".
The difference in intellect between Zack and Aerith is also further emphasized given what happens immediately afterwards. Aerith spends 10 seconds writing the note for Zack, then hands it over and Zack never questions how Aerith supposedly was able to write down 23 WISHES IN A SPAN OF 10 SECONDS. The scene is quite fluid with no indication of pauses that the audience wasn't allowed to see.
Going back to the counting, it's not like Aerith was pulling Zack's leg. She really did come up with 23 tiny wishes, as further evidenced by the note she wrote to Zack.
It is thusly my headcanon that, for example, Aerith always kept a close count in her head of just how many flowers there were in the church. She may even have kept it in memory how many flower petals were present. If a single flower or a single flower petal was missing, Aerith would know it.
In parallel to growing up and feeling different due to her Cetran heritage, Aerith may have felt an uncomfortable disconnect early on when she noticed that others just didn't naturally keep numbers in their head like she did. So beyond a few out-in-the-open Rain Man moments in her early youth, she kept her counting- and memorization talents to herself just so to not freak people out. With Zack however she knew that he was such an oblivious airhead that she didn't have to hide anything. This may be part of why she liked his company so much: She could be herself around him.
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