CK-12 is comprised of free, open access resources for both teachers and students including online textbooks, lesson plans, worksheets, and much more. The site has both a student and teacher view, and on the homepage, subjects are broken down by category. For example, if visiting the probability category under the math subject, lessons plans for counting sets, theoretical and experimental probability, and probability and statistics are available for use in teacher or student view. Lesson plans include activities, videos, practice worksheets, and many include study aids with flashcards. One of the highlights of the site is PLIX, which stands for Play, Learn, Interact, Xplore. PLIX includes interactive activities and is available for all subjects on the site, though using this option does require a free account with CK-12. Also available on CK-12 are its FlexBooks, which are free online textbooks that include text, interactive lessons, worksheets, and embedded PLIX lessons. CK-12 is accessible via a web browser but is also available as an app on iOS and Android devices.