[s] as in "horse" vs [z] as in...

The Big Bang Theory, Season 3, Episode 23, "The Lunar Excitation".

This excerpt is a great example of the [s z] contrast in English. Spanish speakers have a hard time with this contrast because Spanish only has voiceless [s]. Portuguese has both [s] and [z], yet Portuguese-speaking learners of English also have a hard a time with some [s z] contrasts because of the typical positions those sounds might take in a word. 

In  the example below, the [s z] contrast is at the end of the word, and in Portuguese, there is only [s] at the end of utterances (think of "invés" and "talvez", for instance - both have a final [s] if followed by a pause). However, English might have either [s] or [z] in any position of the word, as the example below shows:

If you read Portuguese, you see that the joke is lost in the subtitles, because the punch line of the joke lies in sound similarity of the words, which are contrasted only by the final [s z].

Here are a few more [s z] minimal pairs in Engligh:

[s] [z]
ice eyes
spice spies
loss laws
price prize
face phase
loose lose
once ones
race raise
advice advise
niece knees

Extra explanation 1: Notice that some of the final [z] sounding words above, and the one in the video, are plurals. This is so because when you add the -s morpheme (for plurals, 3rd person singular in the simple present, or the genitive case 's) to a word ending in a voiceless sound, the final -s will be the voiceless [s] (as in cups, stops and Jack's); when you add it to a word ending in a voiced sound, the final -s will be the voiced [z] (as in eyes, plays and Carla's), and when you add it to a word ending in a sibiliant (i.s. [s z ʃ ʒ tʃ dʒ]), the final -s will be pronounced [ɪz] or [əz], with an extra syllable (as in glasses, washes and Carlos').

Extra explanation 2: Another source of difficulty for Portuguese speakers regarding English [s] is the rule that, in Portuguese, the letter 's' between two vowels always sounds like [z], which is not true in English. Therefore, Brazilian learners of English tend to mispronounce the following words, which should all be pronounced with [s] (and not [z], as Brazilians tend to do): house, mouse, blouse, base, case, crisis, basic, useful, fantasy, disorder, disappear, dishonest, misuse, misinform, misinterpret.

Stay tuned for more videos, and if you have a suggestion of a video clip I could use to illustrate a feature of English or Portuguese phonetics, please send it to me.