To foster learner discourse grounded in conceptually relevant content, the instructor chose Hypothesis, an emerging social annotation tool designed to support interactivity virtually on any webpage. Hypothesis functions through a browser plugin or a Javascript-based bookmarklet. After activating Hypothesis in her browser, a learner can annotate a piece of text on a webpage with her own ideas, to which other learners could also respond (Goals 1 & 2). Like other popular social media websites, tags could be used to bring together Hypothesis annotations that are potentially scattered in different webpages (Goal 2). As such, Hypothesis enables multiple entry points of a same discourse (Goal 3). Thanks to Hypothesis' adherence to the Open Web principles and standards \cite{Haslhofer2011-rr}, annotations are uniquely identifiable and retrievable for end-users, allowing portability of posts not widely supported in threaded discussion forums (Goal 2). The search feature on the Hypothesis website offers a means to aggregate annotations under a tag (or a combination of tags) and to review the annotation-based discourse. Participants were thus able to enter the discourse not only from the specific webpage context (e.g., the textbook), but also from a discourse review process on the search page (Goal 3).

In addition, to negotiate the public--private boundary, the designer-instructor allowed course registrants to annotate either publicly or in a private Hypothesis group---both using a same course hashtag (Goal 4). Using this social annotation tool, the class carried on weekly discussions across various web spaces, including the Bookdown course website, the course textbook (accessible via the library's website), open textbooks, journal articles, blog posts, and other web materials shared by the instructor and students (Goal 1). In summary, the social annotation tool enabled a discourse---public and/or private---spread across multiple types of web objects relevant to the course.