How to make this sound:

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The ss and zz sounds are paired together because they have the same mouth position.  Ss is unvoiced, which means you make only air to make the sound, and zz is voiced, you produce sound with the vocal cords.  The position:  the front {top} teeth and the bottom teeth line up together and almost touch.  Ss, zz, ss, zz.  The backs of the teeth are not touching.  They are almost touching.  The tongue position:  the tongue comes up against the back of the bottom teeth and presses very, very lightly.  Ss, zz, ss.  Word pairs:  sip, zip ... dress, says ... Mississippi, Missouri.

International Phonetic Alphabet symbols:  [], []

For more detail see Wikipedia's page on these partnered sounds (voiced and unvoiced).