conuly

Another possible rule is that the apostrophe is only for "longer" plurals, i.e. plurals where the base word is more than 4 letters, so "Chip" and "Food" are too short for an apostrophe. (It can't be based on syllable count, obviously.)

I also wondered if it's based on anything about the class of sounds the word ends with, but I can't find anything. It would have been interesting if it was only after voiced sounds and not voiceless ones, or vice versa, but that doesn't work.