Tkinter: how; newbie

[Gigs_]

can someone explain me this code?

from Tkinter import *

root = Tk()

def callback(event):
     print "clicked at", event.x, event.y

frame = Frame(root, width=100, height=100)
frame.bind("<Button-1>", callback)
frame.pack()

root.mainloop()


well, my problem is at frame.bind(",Button-1>", callback)
callback function prints event.x and event.y.
How the callback function get this two number when it has only one 
argument (event)
Why its not: def callback(x, y): print x, y

Im new to gui programming

Sorry for bad eng!

Thanks for replay!