Interesting real-world examples I found. It was pretty hard to find ones that weren't just masks, but I was pleasantly surprised to see that the hollow-face illusion works in a wide variety of mediums and shapes. It's truly mind-boggling that although I know all these are concave, I can only really perceive the shell as convex.
Is this a good/useful/informative piece of content to include in the project? Have your say!