In many languages there are certain words which have
no singular form, only a plural form. It is
called "plurale tantum", which is Latin for "plural only". Very
often there no particular reason for it and does not conform to
the rules you learn at school.
In English, pluralia
tantum, the plural of "plurale tantum" are usually things that
function together as pairs or sets: scissors, pants, pajamas,
pliers, spectacles, etc.
There is also
"singularia tantum", which are words that have no plural form,
such as information.
There are some examples of pluralia tantum that have become
singularia tantum. One such is 'news', which was originally
plural, but is now always singular. 'Data' was originally the
plural of 'datum', but is now pretty mostly a singulare tantum.
Some words can be used as ordinary nouns or as plurale tantum,
and you can decide which is the most appropriate. For example,
if you want to tell somebody they should apply some thought to a
matter, you can say either "Use your brain" or "Use your
brains". A bad person would want to blow your brains out, rather
than just your brain. There are some that we always get two of,
such as pants. There is one "pant" on each leg, but we never
talk about them separately, because you need both. A pant would
signify just one leg hole.