these words are called 'the inseparable words'
if they are to be used then they cannot be used separately that is why we always refer them as a inseparable words like scissors ,shears, pliers, trousers, sleeves.
and the method of calling them is as follow-
1.Scissors are sharp.( plural verb)
2.A pair of scissors has been purchased.( when using 'a pair of ' then verb will be singular )
3. Your Trousers are very good.( plural verb)
4. your a pair of trousers is good.( when using 'a pair of ' then verb will be singular )