I have seen this lesson everywhere on blogs, so thank you to all of you! I like this lesson as it is a great sub lesson. We discuss optical illusions and look at several examples, the kids love them! Then we draw a circle in the middle of the paper. I show students how to make the lines curve with the circle. I give students a ruler and they draw the grid in the background. We talk about contrasting colors, and the kids are off! When the students are finished coloring, we take vine charcoal and add a shadow to the sphere. I usually mount these on construction paper to make them look finished.