This is the way I think of it and how I always show my students:
A pizza, some pizzas and some pizza:
A pizza is one whole pizza.
Some pizzas are two or more whole pizzas.
Some pizza is just a slice or more of the pizza. It is however much is less than the whole pizza (i.e., less than eight slices).
I usually draw three pictures: one whole pizza, two whole pizzas and a slice of pizza removed (but still near) the whole pizza (don't forget to draw the slice's negative space).
This example also works for cakes, soda (a full bottle, two bottles, some leftover in a bottle), and of course, desserts :).