Nouns can be either singular or plural.

one cat / two cats

The cat is drinking its milk.

There is only one cat in this sentence so cat is a singular noun.

The cats are drinking their milk.

This time there are two cats, so cats is a plural noun.

If you look carefully at the second sentence you will see that two other words have also changed. The verb is has become are and the pronoun its has become their.

When we change singular nouns in sentences to plural nouns we have to make sure that all the parts of the sentence are correct, and not just the noun itself.