Mass nouns are often the names of substances like
water,
wood and
air that we can measure on a continuous scale.
Count nouns, on the other hand, generally label things that come in discrete wholes that we can count like
children,
houses and
hats. To put it simply, the distinction between mass and count is one between stuff and things.
You can usually guess whether a noun will be of the mass or count variety from its meaning, but what ultimately determines whether a word is classified as a mass noun or count noun is the way it functions grammatically.
Mass nouns are always singular, so like singular count nouns, they trigger singular agreement. Mass nouns also have something in common with plural count nouns because continuous substances and groups of countable objects can both vary in quantity. These factors partly explain which determiners you can use with mass nouns and how they compare with the determiners you can use with singular and plural count nouns:
