Given a circle on a paper, and a pencil and a book. Can you find the center of the circle with the pencil and the book?
Let the corner of the book just touch the circle’s edge. You can draw two perpendicular lines along book’s edge which intersect with the circle at two different points. Connect the two points and it’s a diameter.
Repeat this and get another diameter. The intersect of these two diameters is the center of the circle.