Yes, you guessed it! The circle in this image is perfectly round. The straight lines in the background distort the circle so that it appears squashed! If you stare a little while, you can sort of get your eyes to mentally fix the circle, but then it’ll pop back to the distorted shape fairly quickly.
See this other distorted circle. Is the illusion as strong as the one that is in black and white? Why not? This would be a fun classroom project too, where students use their compass to draw a perfect circle. Then have a contest as the students use their rulers to try to distort the circle as much as possible.
Try a few different things with this illusion. First, stare at the center of the circle and see what happens. Then, don’t look at the circle, but pay close attention to it as you scan your eyes around the rest of the image. Last, look at the lines at the bottom and see what the circle does as compared to when you are looking at the bent lines on the right side of this optical illusion!