Because of the space between our two eyes we perceives two distinct images. The difference of the images on our retinas can be easily recognized with the thumb experiment: You only have to put forth your hand, thumbs up. Now, if you first close the right eye and then the left one, the thumb seems to be at different places in comparison to the background (see Figure).
Isn’t it surprisingly, how easy it can be to experiment with human perception without any tools? Here is a very simple one for binocular disparity.