The bell-shaped witch of Maria Agnesi can be constructed in the following way. Start with a circle of diameter a, centered at the point (0,a/2) on the y-axis. Choose a point A on the line y = a and connect it to the origin with a line segment. Call the point where the segment crosses the circle B. Let P be the point where the vertical line through A crosses the horizontal line through B. The witch is the curve traced by P as A moves along the line y = a. In the GeoGebra app below, you can click on the point A with your mouse and move it along the line y = a. This construction of the witch is a reflection of the one mentioned in the biography across the line y = x.