I am in favor of completely open access to educational materials. However, permission is required if you want to use these materials in a commercial or for-profit application.
This material may be used for any non-commercial use by teachers. Please link to this material on your web site or social media site. If you download and distribute any of the materials to your students, please use the following attribution:
© William A. Prothero, Jr, 2011, All Rights Reserved. http://es.earthednet.org/
If you download and modify/improve this material, please send me any significant modifications that you might make, so I can post it on this web site. I will attribute you as a contributor.