Harley, Gail M. and Kieffer, John in 2009 stated that what they knew was Study Tech materials claim that "absence of mass" is the idea that abstractions must be illustrated physically before they can be fully understood: learning about trains is accelerated if the student can see a train or a representation of one. Scientology classrooms are equipped with modeling clay and "demo kits", small collections of everyday objects, such as corks, caps, pen tops, and paper clips. Modeling clay or the contents of such a kit are used to create a physical model of what is being studied, thereby giving the student "mass". One of the course requirements for people learning to be Scientology trainers is to model in clay the premise of every paragraph in Hubbard's book, "".