Pizza, like cake, is both uncountable and countable. Uncounted forms: Let’s have cake. Let’s have pizza. The counted forms refer to the entire circle. A cake. A pizza. You can order two large pizzas and a small pizza. And specific wedges or squares cut from the whole circle are counted using a measure word like slice or piece. There’s nothing grammatically incorrect about any of these.[/quote]
When discussing foods, we learn the convention that, generally, if one or more of the units is likely to constitute a serving, we can discuss the food as a countable noun.
I like grapes, strawberries, hot dogs, apples, bananas, tacos, noodles, perogies, sardines, eggs, and Twinkies.
In each case, the unit will make up a serving or part thereof.
This has a finite minimum, in which the units are so very small as to be served in a practically uncountable mass.
Cereal, popcorn.
Or where the morphology of the food simply disallows counting.
Porridge, polenta, soup, paella, walrus blubber.
Once a food item is so large that a single unit constitutes multiple servings, although the noun remains technically countable, we discuss it as an uncountable when we talk about eating it.
Would you like some watermelon, salmon, pizza, lasagna, papaya, curry (slightly different UK use here, in NA we’d NEVER count curry, but in the UK they do, although you still don’t eat “one”), turkey, chicken, duck, lamb, shark, ham?
AND THEN, when we talk about producing, procuring, purchasing, or preparing them, THEN we use the countable form, as appropriate.
Pick up 2 watermelons, a salmon, 5 pizzas, one and a half lasagnas (tricky, it being a non-English word), a ham, and 6 papayas.
On the farm, my grandma still keeps 10 turkeys, 20 chickens, 15 ducks, 3 lambs, and one big fuckin shark.
And I think it’s pretty much a convention for the last hundred years or so to substitute “a serving of” with “a”, where drinks are concerned.
Whilst, to me, “grab me a water” sounds rather odd, I can see it being used.
A Coke, a beer, a whiskey, a tea, a coffee, all sound fine.