The “"Virtual"” cognitive level involves the use virtual manipulatives in the form of "apps" and "applets" (such as the ones presented in the “Notable collection of virtual manipulatives” below). It involves the virtual representation of the "CPA" cognitive levels in "apps" and "applets". The “Virtual” level involves three "digital-dynamic sublevels": "virtual-Concrete", "virtual-Pictorial" and "virtual-Abstract". This combination of "cognitive levels" provides an alternative way to analyze the development of curricular activities and research studies with a more consistent set of definitions. Wolfram Demonstrations Project  http://demonstrations.wolfram.com/  Wolfram Demonstrations Project contains around 11,000 Virtual manipulatives for math, science and engineering. They are provided in CDF format together with source code. Didax Free Manipulatives Library  http://www.didax.com/virtual-manipulatives-for-math  Didax is the U.S. branch of Philip & Tacey, Ltd of Hampshire, UK, who developed Unifix₢ Cubes in 1960, a popular math manipulative used throughout the world to teach counting and operations. Unifix cubes were created as a replacement for poppet beads, which rolled off student desks and were expensive to manufacture. The virtual manipulatives in this library are designed to be faithful to their physical counterparts and include minimal navigation or symbolic content. Shodor Interactivate Activities  http://www.shodor.org/interactivate/activities/  Shodor is a national resource for computational science education. They have offered online education tools such as Interactivate and the Computational Science Education Reference Desk (CSERD) since 1994. The activities are sorted from Grade 3 through Undergraduate. National Library of Virtual Manipulatives  http://nlvm.usu.edu/  Utah State University has offered this collection of internet-based manipulatives since 1999. The activities are sorted from Pre-Kindergarten through High School. The manipulatives were originally developed in Java. Illuminations: Activities  http://illuminations.nctm.org/Default.aspx  Illuminations has been found on a section of the website for the National Council of Teachers of Mathematics since 2000. Students and teachers from Pre-Kindergarten through High School can use these interactivities.