I'm developing a simple game by andengine. I have 10 balls which are moving randomly on screen.i'm importing the balls as picture in sprites.if they move at the same coordinate , they pass though their own insides.but i want: if they move at the same coodirnates ,they should change their directions.so they cannot pass through their insides.how can i do that?

I'm developing a simple game by andengine. I have 10 balls which are moving randomly on screen.i'm importing the balls as picture in sprites.if they move at the same coordinate , they pass though their own insides.but i want: if they move at the same coodirnates ,they should change their directions.so they cannot pass through their insides.how can i do that?

 private Runnable mStartCircle = new Runnable() {
  public void run() {
    int i = circleNumber++;
    Scene scene = Level1Activity.this.mEngine.getScene();
    float startY = -64.0f;
    float startX = randomNumber.nextFloat()*(CAMERA_WIDTH-70.0f);
    float a= randomNumber.nextFloat()*(CAMERA_WIDTH-70.0f);
    circles[i] = new Sprite(startX, startY, textRegCircle[i]);
    circles[i].registerEntityModifier(
            (IEntityModifier) new SequenceEntityModifier (
                        new MoveModifier(10.0f, circles[i].getX(),  a, 
                                circles[i].getY(),CAMERA_HEIGHT+64.0f)));
    }
    scene.getLastChild().attachChild(circles[i]);
    if (circleNumber < 10){
        mHandler.postDelayed(mStartCircle,1000);
    }
}

};