This month's free project on the Math Forum shows how to start with a circle, add some more circles and construction lines . . .
. . . and create a dodecagon.
Once you have your dodecagon, you'll also see some cool things you can do with it:
The Student Version has a question about the number of circle "sides" needed to start with, to create a dodecagon. The Teacher Version has the answer! Enjoy.
Anyone can design anything in 3D! http://www.3dvinci.net/