I find this cool: it is very easy to draw a circle with a steel square!
I have tried it with a piece of cardboard cut to shape (this was my steel square ;-)) and 2 pins and it did indeed work :)
Links:
Draw a Circle with a Steel Square (Carpenter Tools for Measuring and Marking)
=> scroll down to Make a Circle With a Square
English Wikipedia entry for ‘Steel square’
German Wikipedia entry for ‘Winkel (Werkzeug)’