why does s sounds like /s/ in "cats "& like /z/ in "fans"?

Answer 1

jurydoc is correct, but in case that didn't tell you enough:

/s/ and /z/ are related sounds --the same, except /z/ uses the voice, /s/ does not.

Now when you form a plural by adding an "s" (also to form singular verbs), it 'follows the lead' of the sound just before it on whether of not to use the voice,.

After 'voiced' consonant sounds s (b, d, g, l, m, n, r, v) and vowels (which ALWAYS use the voice in English), the "s" ends up being voiced -- making the /z/ sound.

If the preceding sound is NOT voiced (f, k, p, t) the "s" will make the /s/ sound (without the voice).

(Notice that I left certain consonant sounds out. That's because some of them CANNOT appear before the added s. S and z, for example force us to add a vowel in between to be able to pronounce it.)

Answer 2

Because of the consonant that precedes it.