Method 1: Fold the circle in half once, and the crease is the diameter of the circle. Fold again to get another diameter, and the intersection of the two diameters is the center o;

Method 2: Make two non-parallel chords of a circle, and then make a vertical line in the middle of the two chords. The intersection of the two vertical lines in the middle is the center o.

Method 3: take any three points on the circle, and then connect these three points in turn to make the middle vertical line of AC and AB, and their intersection point is the center of the circle.