Seriously, this has haunted me since elementary school! i Never understood what I was supposed to do, and now that I know HOW to do it, I still am not able to! So frustrating.
I haven’t seen one in years though. When I was younger I used to be really good at it – I used to even look at other things such as wallpapers or designs the way you look at stereograms and even those things would seem to pop out! Good luck.