Here's why:
* General Rule: For most nouns, you add an apostrophe and an "s" to make the possessive form (e.g., dog's, cat's, book's).
* Exception for Nouns Ending in 's': If a noun already ends in "s," you can add just an apostrophe (e.g., James's, or James' for a less formal style). However, for the word "glass," adding "s's" is the more common and grammatically correct form.
Example:
"The glass's surface was smooth and clear."