Linguistics Help? Distinguishing between s and z?

Basically, both s and z are a hiss between your teeth. But s is what's called "unvoiced," which means you don't feel your vocal chords vibrating when you say s, it's just a hiss. But when you say z, it's "voiced," meaning you vibrate your vocal chords. Hold your hand over your throat and say both, you'll see (or rather, feel) what I'm talking about. That should help you with your homework.