Subject: how do i make a 2d image appear 3d

a few years ago i saw an exhibit in a science museum that at first glance 
all you saw were two identical pictures side by side and then when you 
looked through a pair of glasses that were fixed above the pictures 
everything appeared to be 3 dimensional.  from what i remember my left eye 
was seeing the left picture and my right eye was seeing the right 
picture.  after playing with this idea for a little while i am still lost 
and am wondering how they made it look 3d.  as i said it was about 5 years 
ago and nobody at the museum seems to know what i am talking about now. so 
i was just wondering if you could tell me if it is possible for me to do 
the same with some pictures i have.  thank you for any help you might be 
able to give.

Re: how do i make a 2d image appear 3d