Singular Possessive Nouns

Possessive nouns show ownership. Singular possessive nouns are singular nouns that have ownership of something else.

dog's collar (the collar that belongs to the dog)
rainbow's colors (the colors that belong to the rainbow)
doctor's friendliness (the friendliness that belongs to the doctor)

To make any noun possessive, write the noun first, and then add an apostrophe to the end. If there is no -s at the end of the noun, add an -s. Do not add an apostrophe to a word unless the word owns or possesses the word right after it.

I filled my dog's bowl with food. (The bowl belongs to the dog)
My mom's friend is coming over for dinner. (The friend belongs to my mom)
The cat's fur is orange and white. (The fur belongs to the cat)

Incorrect: All the girl's in the classroom have pink backpacks. ("in" does not belong to "girl")
Incorrect: My brothers got new phone's yesterday. ("yesterday" does not belong to "phone")
Incorrect: There was a special lunch for the teacher's at school. ("at" does not belong to "teacher")