In this illusion, you can see a distinct rectangle in the center and you can see a gradient of grey as the background. You can't deny the fact that you can see a gradient even in the center rectangle which is going from the lighter shade to the dark shade from left to right. But will you do one thing? Take this image and go to any image editing software. Now sample the different regions of that rectangle. You will be shocked to find out that the grey rectangle in the center does not comprise of any gradient and is rather a same shade throughout. So why does it appear so? This is because of the gradient in the background which makes us perceive what we are.