public void HandleCollisions()
        {
            // handle on screen collision

            // object collision
            foreach (ICollidable objMover in collidableObjects)
            {
                // first object must be moving
                if (typeof(IMobile).IsAssignableFrom(objMover.GetType()))
                {
                    IMobile mover = (IMobile)objMover;

                    if (!mover.Stopped)
                    {
                        // scan all other objects to see if there is a collision with them
                        foreach (ICollidable objCollider in collidableObjects)
                        {
                            if (objMover != objCollider &&
                                objCollider.CurrentObjectRectangle.Intersects(objMover.CurrentObjectRectangle))
                            {
                                // detect collision direction
                                CollisionDirection direction = CollisionDirection.NONE;

                                if (objMover.CurrentObjectRectangle.Bottom > objCollider.CurrentObjectRectangle.Y &&
                                    objMover.CurrentObjectRectangle.Bottom < objCollider.CurrentObjectRectangle.Bottom)
                                {
                                    if (objMover.CurrentObjectRectangle.Right > objCollider.CurrentObjectRectangle.X ||
                                        objMover.CurrentObjectRectangle.Left < objCollider.CurrentObjectRectangle.Right)
                                    {
                                        direction = CollisionDirection.TOP;
                                    }
                                }
                                else if (objCollider.CurrentObjectRectangle.Bottom > objMover.CurrentObjectRectangle.Y)
                                {
                                    direction = CollisionDirection.BOTTOM;
                                }

                                // handle collision on the mover
                                if (typeof(ISurface).IsAssignableFrom(objCollider.GetType()))
                                {
                                    mover.HandleMobileCollision <ISurface>(objCollider, direction);
                                }
                            }
                        }
                    }
                }
            }

            // game area edge collision


            // TODO : defer off screen collision to another Thread
        }