An amazing live-action demonstration of the checker shadow illusion, where colored squares are not quite what they seem. As spooky as watching the colors change before your very eyes, it’s got nothing on the actual explanation which shows how this trick is deceptively simple. From the YouTube description:
[...] within the shadow region the brightness and darkness of the squares are still relative to one another. But they are no longer relative to the squares that lie outside of the shadow.
So if you were able to pick up the middle square and move it to one of the outside dark squares, they would be 2 different shades, But the trick is [...] when you move the square you are moving it WITH the shadow still printed on the square.
That’s the last time I ever trust my own eyes.