When the author describes how two things differ, he or she is _____ them.

When an author focuses on how two things differ, they are engaging in the process of contrasting. This involves highlighting the distinctions and differences between the two entities, rather than their similarities or overall characteristics. The act of contrasting is crucial in literature and analysis, as it allows readers to see the unique features that set each item apart, helping to clarify their individual significance or role within a broader context.

Unlike comparing, which emphasizes similarities and parallels between two subjects, contrasting draws attention specifically to divergence. Summarizing, on the other hand, involves concisely restating the main ideas or points of a text without necessarily focusing on either differences or similarities. Comprehending relates to understanding, which is not the primary focus of the question regarding differences. Hence, when the context is about identifying how two things are not alike, the term contrasting fits precisely.