Look through the tubes at the white screen, wall, or paper. Overlap the spots. Switch the tubes and repeat. If you saw only the circle before, you may see the oval now. Your eyes and brain have trouble merging the different shapes.
Most people have a dominant eye.
The brain will choose to see the image that is coming from the dominant eye. Some people do not have a dominant eye, and therefore see the two shapes overlapped. The best baseball hitters do not have a dominant eye.