Underneath their compelling audiovisual surface, games require players to carry out mundane interaction work, such as pointing, typing, or steering. However, many of these underlying building blocks are not defined rigorously, hampering synthesis and analysis. We elaborate on the origin of tasks within human-computer interaction (HCI) and define tasks' relationship to game terminology (game mechanics, goals, and actions). Our proposed framework draws on systemic-structural theory of activity to aid systematic analysis and exploration of game design by mapping gameplay to abstract core tasks. The framework contains four task tools, applicable when 1) uncovering design properties, 2) designing experimental manipulation, 3) creating behavioral measurements, and 4) describing gameplay in literature reviews of game genres and design techniques. We evaluated our framework as a lens to design purposeful games in three case studies within a scientific education. We invite researchers and practitioners to employ the framework as a microscope, to describe and design games rigorously.