"A glass of water."
Translation:Un vaso de agua.
They are saying that Taza is "cup" not "glass". I think this is being overly picky.
Literally just how each is said in English. It's common to say "a glass of water", not "a water glass" (the latter would read like a glass for water but not necessarily with any water in it), while it's common to say "a meat sandwich" rather than "a sandwich of meat".
Basically, this is just English being weird.
It stems from the French translation as in many cases;
Un vaso de agua (Spanish) Un verre d'eau ( French) A glass of water (English)
Pretty much the same to me!
Sorry for the beginner's level question, but having completed the French course, I was wondering if Spanish ever contracts as with French. In this particular sentence the two last words would contract to "d'agua". Looks funny even writing it, actually...