Expii functions as an interactive series of math and science textbooks. Po-Shen Loh, a math professor at Carnegie Mellon University, created Expii with the aim of making math and science instruction more interactive and collaborative. The resource can be navigated in one of two ways: users may either search for a specific topic, such as "quadratic equations" or "Newton's first law," or they may use the Topic Map for game-like exploration. This second approach reveals the many connections between math and science and allows users to create their own journey through a variety of subjects, such as algebra, physics, astronomy, and calculus. Topics are accompanied by short explanations of concepts and often include complete practice problems. Notably, Expii is a crowdsourced learning site; contributors from around the globe can add their own definitions, create new problems, or update problem explanations.