Imagine a textbook in which the illustrations are dynamic so that a pendulum, for example, actually swings. Further imagine that the reader has the ability to choose what view of the pendulum is displayed. It might be an animation view representing the movement of the pendulum in real space, it might be a graph of angular position and velocity vs time, it might be a plot of position vs velocity. Now suppose that the reader could change the length and mass of the pendulum, include bearing friction and air resistance and push the pendulum in several different ways. In effect a virtual model of the pendulum would sit there on the page surrounded by text detailing the theory and practice of swinging objects.

Now let's imagine that in this textbook the text may be edited, added to or written from scratch by the reader and that the new physical systems may be created to illustrate the principles expounded in the text... and that these new systems may be studied and modified by future readers. This is the paradigm upon which M. Casco Associates has based a series of run-time books, including the virtual laboratory in the book as embedded models.

The run-time terminology comes about because with these books you have the ability to alter the program while it is running. This gives you unprecedented interactive power. Rather than just choose your path through material that we have written, you change the characteristics of the modeled system to explore its behavior under conditions you choose. You can even create entirely new systems beyond the imagination of the original author.