Stage: 2 Challenge Level: 
You could try using twelve sticks of equal length such as headless matches.
Addition & subtraction. Working systematically. Regular polygons. Interactivities. Practical Activity. Visualising. Compound transformations. Investigations. Combinations. Mixed triangles.
Published January 2000,January 2009.
Copyright © 2007. University of Cambridge. All rights reserved.
NRICH is part of the family of activities in the Millennium Mathematics Project, which also includes the Plus and Motivate sites.
email: nrich@damtp.cam.ac.uk