The verb "fly" is an irregular verb, and its third-person singular form in the present tense adds "-es" at the end. This is done for verbs that end in "-y" and the letter before the "-y" is a consonant. Therefore, "fly" becomes "flies" when referring to a third person, such as "he flies" or "she flies."
This rule applies to other verbs as well, like "try" (tries), "cry" (cries), or "study" (studies). It's important to know these verb forms to communicate accurately and correctly in English.