fruit:只表示"水果"是不可数的
There is some fruit on the table.
如果表示水果的"多种多样"那就是可数的
All kinds of fruits are available,including apples,pears,peaches,bananas and oranges.
food：表示单纯的食物时不可数
比如
I'm hungry.I want some food.
表示几种食物时可数
We have a lot of foods.Fish,meat,chicken,which one would like?