The distance between the rulers doesn't change and so they behave just a circles would even though they are not circles! How is this possible? There are loads of videos and explanations about this on the internet if search for 'Shapes of constant width'. Go searching and have some fun playing with this idea. A great place to start might be http://howround.com/ and follow the link to 'How round is your circle' where you can see some geogebra applets that help you figure what is possible. Please add links to any cool related videos you find in comments on this blog post.
Good luck and have fun!
