Ensuring Pronoun-Antecedent Agreement
The word that a pronoun replaces is called its antecedent. If the antecedent is singular, the pronoun must be singular. If the antecedent is plural, the pronoun must be plural. They must agree in number.
The two friends performed for their classmates.
(Because the noun "friends" is plural, the pronoun "their" must be plural.)
Kate held binoculars up to her eyes.