Optional material may vary in length from bibliographical citations and brief comments to discussions extending over several pages. Often, such "excursions" provide more detailed elaboration of the main line of reasoning, supporting mathematical derivations, responses to likely questions, practical examples illustrating the significance and wide application of the principles under discussion, or recommended readings from online or printed sources.

Each excursion concludes with a "Return" button like the one below. In order to avoid confusion, it is recommended that this button be used to return to the main course, and that the close button at the top of the window should not be used until the student is ready to terminate his or her session and to exit from the primary course window. To return to the main course now, press the "Return" button.