3D view by crossing eyes

The required distance from the image to your eyes will vary, depending on the image size and individual's eyesight. Choose the distance at which you can comfortably see sharp image and look at it for few seconds

Hold a pen close to your face and try to focus your eyes on it's tip

Slowly move pen away, towards the centre of the image

At some point, about 20-30 cm away from you, you will start noticing, that in the background image the third picture appearing in the middle, between two original images

Continue to move the pen until the central image becomes clear and separated from original two images

Hold the pen there for few seconds, trying at the same time to re-focus onto central, combined image and slowly remove the pen from the field of view.

Practice on this simple image: