Open Education Resources
Open Educational Resources (OER) are teaching and learning materials that reside in the public domain or have been released under an open license. These resources may be used free of charge, distributed without restriction, and modified without permission.
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How to find Open Education Resources with Google
Type your topic into the Google Search box. After your search results populate, click on Settings to activate the pull-down menu.
Select Advanced Search from the pull-down menu.
Scroll down to Usage Rights and select free to use or share (or the level that meets your needs).