Optical illusion of a green grid on a greyish mosaic background. Although by concentrating on small segments at a time you realise the grid is made of straight lines only, the lines appear to curve elsewhere. It's caused by our common inability to focus in our whole field of vision
https://media.beige.party/media_attachments/files/113/090/752/786/465/906/original/e5cfd73ea7e88365.png