CK-12 Foundation is a leading open educational resources (OER) non-profit organization dedicated to increasing access to high-quality K-12 STEM education materials. CK-12 offers free high-quality, standards-aligned, open content in the STEM subjects through an integrated set of tools for learning including digital textbooks, concept-based learning resources, simulations, interactive practice and more. All content is created and curated by teachers, for teachers and students. All CK-12 products and services are 100% free and are being used by students and teachers across thousands of schools both in the US and rest of the world.