How do I know when words are written with “z” or with “c”?

In Spanish, there is always a constant rule to know whether to write words with ‘z’ or ‘c’:This only applies for the ’C’s and ‘Z’s that both phonetically sound like “th”. The rule is: za ce ci zo zu. You can apply these to any case since they don’t vary, for example, try saying these out loud and distinguishing their phonetical sound:Cecina (animal fat, usually pig)- here we can se how both the rules of “c” have been applied. Cebra (zebra the animal)- Cabeza (Head)- Zoológico (a zoo)