Geologic time is a central concept in Earth science teaching and research, but undergraduate students are seldom involved in doing time: measuring 'ages' of rocks or minerals. Integration of apatite fission track (AFT) analysis in geoscience classes is an accessible way to involve students in determining dates for geologic events. AFT analysis is an active-learning, inquiry-based technique in which students can visualize and understand concepts such as radioactive decay and its relationship to geologic ages, and the method and its applications can be a basis for cross-discipline instruction within a physical sciences curriculum. Fission-track analysis exercises can be incorporated into a variety of physical science classes or can be run as a separate workshop on geologic time or tectonics. We present a teaching module for incorporation of AFT analysis into general physical geology or historical geology lecture and/or laboratory. This exercise involves counting fission tracks using images from prepared samples (apatite grains already separated from a rock) with known uranium content. The data collected by students can then be used to demonstrate a variety of concepts, from radioactive decay to time-temperature paths and rates of geologic processes. With this module, undergraduates learn problem-solving techniques and experience a hands-on, quantitative approach to geologic time, rates of geologic processes, and the scientific method in general.