Yes that top reddit comment is right. Years ago those magic eye pictures came out and at first I struggled to see the 3d pictures. Eventually it became trivial. Then somewhere I noticed it worked really well for those spot the difference puzzles. They often come on kids menus at some restaurants. I could spot all ten differences in seconds and my kids would be amazed. I finally tried to teach them but they found it hard to do. But ya in reality what this girl did was super easy and not challenging in the slightest. When you do it and cross your eyes inwards you cross your eyes until the pictures line up in the middle. The majority of the picture will be clear but any spots that are different will seem blurry. The trick is just not focus on anything in particular and just cross images until they clear up. Then make note of where it seems blurry. Cool trick but definitely ruined spot the difference puzzles for me.