Geometry: using only compasses and straightedge
Make a dot. Draw a circle round it using compasses.
Draw a line from one point on the circle through the centre dot to the orther side of the circle. This is a diameter. It divides the cicle in two.
Draw another diameter through the centre at right angle (perpendicular) to the first diameter. (How to draw a perpendicular?)
Join the four ends of the diameters. You have drawn a perfect square.
Draw a square inside a circle.