The eye works by letting in the correct ammount of light through the pupil which then hits the back of the eye. Special cells called rods and cones read that light and then transmit the information via the optic nerve to the brain.
On the first can they placed tinfoil over the opening and poked a small hole in it. This small hole acted as the pupil and let light in. On the second can they placed a plastic shopping bag over the opening. This acted like the back of the eye.
By looking through the cans, children were able to see what their eyes see.
