Watermelon is not plural in * I am making a fruit salad. I am mixing mangoes, watermelon and oranges.* because it’s a mass (uncountable) noun.

Many fruits and vegetables are mass nouns: lettuce, spinach, watermelon, cantaloupe, avocado.

But if you’re shopping, you might buy two watermelons, three avocados, a head of lettuce, a bunch of spinach.

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Watermelon is not plural in

I am making a fruit salad. I am mixing mangoes, watermelon and oranges.

because it’s a mass (uncountable) noun.

Many fruits and vegetables are mass nouns: lettuce, spinach, watermelon, cantaloupe, avocado.

But if you’re shopping, you might buy two watermelons, three avocados, a head of lettuce, a bunch of spinach.