Заставить шар лететь обратно

from tkinter import *
 
WIDTH = 900
HEIGHT = 300
BALL_RADIUS = 30
 
 
root = Tk()
root.title("PythonicWay Pong")
 
c = Canvas(root, width=WIDTH, height=HEIGHT, background="#003300")
c.pack()
 
 
 
 
c.create_line(PAD_W, 0, PAD_W, HEIGHT, fill="white")
 
c.create_line(WIDTH-PAD_W, 0, WIDTH-PAD_W, HEIGHT, fill="white")
 
c.create_line(WIDTH/2, 0, WIDTH/2, HEIGHT, fill="white")
 
 
 
 
BALL = c.create_oval(WIDTH/2-BALL_RADIUS/2,
                     HEIGHT/2-BALL_RADIUS/2,
                     WIDTH/2+BALL_RADIUS/2,
                     HEIGHT/2+BALL_RADIUS/2, fill="white")
 
 
BALL_X_CHANGE = 20
 
BALL_Y_CHANGE = 0
 
def move_ball():
    c.move(BALL, BALL_X_CHANGE, BALL_Y_CHANGE)
 
def main():
    move_ball()
    # вызываем саму себя каждые 30 миллисекунд
    root.after(30, main)
 
 
main()
root.mainloop()

from tkinter import *
 
WIDTH = 900
HEIGHT = 300
BALL_RADIUS = 30
PAD_W = 20
 
root = Tk()
root.title("PythonicWay Pong")
 
c = Canvas(root, width=WIDTH, height=HEIGHT, background="#003300")
c.pack()
 
c.create_line(PAD_W, 0, PAD_W, HEIGHT, fill="white")
c.create_line(WIDTH-PAD_W, 0, WIDTH-PAD_W, HEIGHT, fill="white")
c.create_line(WIDTH/2, 0, WIDTH/2, HEIGHT, fill="white")
 
x = WIDTH/2-BALL_RADIUS/2
y = HEIGHT/2-BALL_RADIUS/2
 
BALL = c.create_oval(x, y,WIDTH/2+BALL_RADIUS/2,
    HEIGHT/2+BALL_RADIUS/2, fill="white")
 
BALL_X_CHANGE = 20
BALL_Y_CHANGE = 0
 
def move_ball():
    global x, y, BALL_X_CHANGE
    x += BALL_X_CHANGE
    y += BALL_Y_CHANGE
    if x > WIDTH or x < 0:
        BALL_X_CHANGE *= -1
    c.move(BALL, BALL_X_CHANGE, BALL_Y_CHANGE)
 
def main():
    move_ball()
    # вызываем саму себя каждые 30 миллисекунд
    root.after(60, main)
 
 
main()
root.mainloop()