linguistics1-3  Exercise 2: The English Plural
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  Noun    Final sound   Plural ending   Plural
  ------  ------------  --------------  --------------
  cat     [t] voiceless  [s]            cats   [kæts]
  dog     [ɡ] voiced     [z]            dogs   [dɒɡz]
  bus     [s] sibilant   [ɪz]           buses  [bʌsɪz]
  cab     [b] voiced     [z]            cabs   [kæbz]
  cup     [p] voiceless  [s]            cups   [kʌps]
  church  [tʃ] sibilant  [ɪz]           churches [tʃɜːtʃɪz]
  bee     [iː] voiced    [z]            bees   [biːz]

Rule:

  - After a voiceless sound (t, p, k, f, θ), the ending is [s].
  - After a voiced sound (including all vowels), the ending is [z].
  - After a sibilant (s, z, ʃ, ʒ, tʃ, dʒ), a vowel [ɪ] is inserted and the
    ending is [z]: [ɪz].

Invented words:

  blick  ends in [k], voiceless   ->  blicks   [blɪks]
  zudge  ends in [dʒ], sibilant   ->  zudges   [zʌdʒɪz]

These predictions are what nearly every English speaker will say, without
ever having been taught the rule. That is the evidence that the rule is part
of the speaker's own phonology.