The German word "Wortschatz" literally means "treasure of words". It is used to describe the entire amount of words a person may know and use in their daily language.
And actually, I think it's very interesting how each and everyone of us has some other "treasure of words".
We all use words to express what we're thinking, feeling or simply doing. Most of the time, we don't spend a moment on thinking how exactly we express ourselves or which words we should use. But yet, everyone has their own favorite phrases or habits. Everyone expresses themselves through words, another one would never think of saying. Our choices (mostly not even knowingly) of words are part of who we are because these words are the way we choose to communicate with the world. To me that's the reason why words themselves are an actual treasure. They are the translator of our complex thoughts into an acoustic signal through which we are able to share our mind with others. We are hardly ever aware of that, but I think, that's also the reason why we should pick our worlds wisely in every situation.
