In English grammar, verbs change their forms depending on the subject of the sentence. In the case of "fly", when the subject is a singular third person, such as he, she, or it, we add an -s to the base form of the verb.
For example, we say: "He flies to different countries every week" or "The bird flies gracefully in the sky".
Adding the -s to "fly" is a common rule in English for forming the third person singular present tense. This rule applies to many other verbs as well. However, it's important to note that some irregular verbs may have different forms in the third person singular.