English and German are both Germanic languages. They have a common ancestral language, Proto-Germanic. As a result, it's not surprising that there are many similarities between them. In a number of words an English t corresponds to a German z.
Here are examples of English words with a t and German words with a z:
The number of words in which an English t corresponds to a German z indicates that the two languages are related. In Proto-Germanic these words were the same. Historical linguists believe that the German z of these words developed from the t which is still preserved in English.