An element is just a certain kind of material part. Consequently an element is related to that of which it is an element in the same way that part and whole are related in general. As was said in the linked post, the whole is simply speaking not the part, but that which is the whole exists as that which is the part; or in other words, that which is the whole is, in a certain respect although not simply speaking, that which is the part.

This is why not all things are water, namely because the whole which is composed of elements is not the elements, simply speaking, but is those elements only in a certain way.