Let's hear it!
Ok... Let's see if I can keep this short....
It's a play on our English mnemonics, "A is for apple, B is for ball," etc. In many languages with "modern" writing systems (<1500y), the letters tend to be phonetic or phonetic ish, with consonants being a mix of the consonant sound and a random vowel (ef, jee, aech), or something a bit more structured like the Slavic / Cyrillic / Esperanto letter-plus-specific-vowel. (In Esperanto, it's Ah, bo, tso, cho, do, eh, fo...)
If you go back to the older written languages, the letters have names. Greek is a good example where the names have started to loose their meaning. If you ask someone from Athens or Thesaloniki what "alfa" or "vita" means, they will say "they're letters".
If you go back a little further, that changes. The names for the older Semitic languages like Arabic ('alif, baat), Coptic (alpha, veeta), and Hebrew (aleph, beth) used to mean something. If you study old Hebrew, you learn not only the names for the letters, but what the names mean.
This is because the original pictograms (which later lead to cuniform, stylized hieroglyphs, Linear A & B, Cypriot, Nabatean and Palymerian (the writings from which Arabic and block & script Hebrew drew quite heavily), and even a few Shang dynasty clavacle inscription characters) were influenced by the rebus principle. The character for the glottal stop was an ox head, because the early Babylonian/Akkadian/Proto-Semite word for "ox" started with a glottal stop.
When the inventory of "sound-characters" was ordered, it's thought that the glottal stop was placed first because it wasn't really a sound, but a stop, and is most common in Semitic languages in word-initial placement (I know it occurs anywhere, but it's common as word-initial).
The old glottal stop character looks a lot like a capital letter A, but with a longer cross bar, and rotated 180deg (upside down). However, in many monumental forms of writing, it became popular to turn writing so it became vertical columns. (If you have ever seen cuniform cylinders, they are read top-to-bottom, but were usually inscrbed sideways.) So, the characters were rotated counter-clockwise 90deg. If I remember correctly, the Syrian alif still looks a lot like this, but with a squared-off point.
Then, the Phonecians rotated the letters again. The Phonecians encountered the Cyprians and Minoans (who had their own forms of writing), but the Phonecian form made it to mainland Greece, and was re-tooled to match their aesthetics.
So, the ox head was turned upside-down.
A is for Ox.