Do we really need two cameras for 3D vision? I heard that we can see the world 3D or we can detect distance because we have 2 eyes. but I think we can do it with only one eye! (test it!)

So why we can do it only with one eye? is it possible to have stereo vision for a robot only with one camera?
First of all, our brain does not only rely on our stereo visionary system to estimate the depth. There are many cues in a image scence for depth estimation, of which stereo, vision belongs to a sub-type called Binocular cues. 

Technically there are many other methods of depth estimation, like Structure from motion, Perspective,etc. Just take a look at here and here.