The key answer of exercise 21

My friend Marco enjoys collecting optical illusions, so I’ve promised to help him. He wants to create a collection and publish the best in a book, so I agreed to spend a few hours checking out the internet yesterday. I found some great ones, but I was so tired I forgot to send them. So I have to do that later today. Some of them are difficult to see. Marco suggests relaxing with closed eyes for a minute before looking at them, and that helps. You have to avoid being too stressed when you look at them.