Here's why:
* Singular: When referring to the item itself, like a pair of glasses, it's singular.
* Plural: When referring to multiple pairs, it's plural.
Examples:
* Singular: "I need to get new glasses." (Referring to one pair of glasses)
* Plural: "The glasses on the table are mine." (Referring to multiple pairs of glasses)
Let me know if you have another example in mind and I can help you determine the correct form!