go方法中怎么有那么多括号，有点晕

package main

type Teacher struct {
    age int    
}

func (teacher Teacher) showAge(age int) (int) {    //怎么这么多括号，看得晕
    teacher.age=age
    return teacher.age
}

func main() {
    var teacher Teacher
    var r=teacher.showAge(26)
    println(r)
}