Concentrate on the center point of the circle and you will see that it starts moving as well as expanding. Actually it the lines are the same and are not moving.This is just an illusion to your eyes
OK, this time you need to cover your right eye and stare at the black circle. The move closer to the screen and see the cross disapear. Move back again and the cross will reappear...
Now imagine the cross is a motorbike and think about them next time you go out in the car!