I made this hand print for my granddaughters on August 15, 2019. I took one picture when the cement was still wet and another one when it was dry. All the people I showed to see the wet picture as a convex hand print. Some people were able to see the concave image after rotating the page. As for the dry picture, people saw the concave image at first but then quickly turn into a convex one especially turn it 90 degree clockwise. Once one saw the convex image, most of them would’t be able to see the concave image anymore no matter how you rotate it. For the experiment, look at one picture at a time first. After that, flip through or see all 4 pictures in the same screen. You most likely would see two concave and 2 convex images. All photos were taken with mobile phone and haven’t been edited. After you have seen convex image, scroll backward to see if you can still see the same images.
Keep playing with individual image and you will see all the different result. It is simply amazing. Share if you like.