With it set on A, half press the shutter whilst looking into the lens. You should see the aperture change. Try it in bright light and in your own shadow. In bright light it should be closed down and in shadow it should open up.
This camera has a meter with a needle which you cant see. When you half press the shutter, the mechanism for moving the aperture blades jams up against the needle with an angled plate. The amount of light dictates where along the angled piece the needle rests and therefore dictates the aperture opening.