There are certain words in English which are always used in the plural form. For instance, Spectacles. Since we have two eyes ,two spectacles are always required. Therefore using the singular form(Spectacle) of this word will be wrong. Similarly there are other words which follow the same rule.
1 Trousers- Since we have two legs.
2 Scissors – Since it has two parts.
3 Tongs- Since it has two sections.