Geometry: using only compasses and straightedge
Draw a square outside a circle.
Make a dot and draw a circle round it using your compasses.
Draw a line from one edge of the circle through the centre to the other side. (A diameter, it halfs the circle.)
Draw another diameter at right angles (perpendicular) to the first diameter. (How?)
Draw lines perpendicular to A, B, C, D.
Join where the perpendiculars meet. You have a square outside a circle.