Some nouns end in s even though they are considered singular. These include words such as mathematics, civics, measles, news, physics, mumps, and molasses.
Physics is my favorite class.
The mumps is a very painful illness.
Other nouns that end in s, such as scissors, acoustics, pants, trousers, and shorts, are plural and always take a plural verb.
These scissors are mine.
Josh's pants are getting too short.
If the phrase pair of comes before a noun ending in s, always use a singular verb, even if the noun is usually plural.
This pair of pants fits perfectly.
There are some nouns ending in s that can be singular or plural. Some examples are series, species, means, chassis, corps, and headquarters.
This species of fish lives in fresh water. (just one species)
These species of fish live in salt water. (many species)