为什么GoLang方法会产生编译错误？

I am a trying the Methods in GoLang. I am a newbie so please correct me if I am asking dumb question. The link says that we can write the Methods as normal Functions. But when I try following code it gives me compile error as

a.sq undefined (type MyFloat has no field or method sq)

The commented lines in following code are working as expected though. Please help me. Following is my code:

package main

import (
    "fmt"
)

type MyFloat float64

func sq (f MyFloat) string {
    return fmt.Sprintln("The square is: ", f*f)
}

/*func (f MyFloat) sq() string {
    return fmt.Sprintln("The square is: ", f*f)
}*/

func main() {

    a := MyFloat(2.0)
    fmt.Println(a.sq())
}

You are declaring sq as a function, not a method. If you want to attach sq to MyFloat, you should declare it like:

func (f MyFloat) sq() string {
    return fmt.Sprintln("The square is: ", f*f)
}

This way you will be able to do a.sq().