Addictive viewing! Can't wait to show my high school math students. Worth every cent of the 70 cents requested, plus many more coins as well! I, too, had trouble with entering the registration code. The developer responded - "To enter the code, press R whilst the software is running. It should then switch to the registration window. Then, without doing anything else, type in the registration code using the top row number keys." This worked for me. Great job. Use as a screen saver would be nice.