Ahhhh brings back all the memories - or frustration - depending on whether you could do them or not haha!
Bonus points if you can do these on a tiny mobile screen! Haha.
If you can't do them, here's some tips on how from their website:
Move the image close to your nose (and eyes) - or move towards it. It should be blurry. Focus as though you are looking through the image into the distance. Very slowly move the image away from your face until the two squares above the image turn into three squares. If you see four squares, move the image farther away from your face until you see three squares. If you see one or two squares, start over!
When you clearly see three squares, hold the page still, and the hidden image will magically appear. Once you perceive the hidden image and depth, you can look around the entire 3D image. The longer you look, the clearer the illusion becomes. The farther away you hold the page, the deeper it becomes. Good Luck!