This online game tests reaction time. The learner controls a computer-generated frog, and must press a button as soon as a fly appears on the game's radar screen or can be heard from the speaker. A sequence of six events that go into this particular reaction time are listed for discussion with learners, analyzing the human nervous response to some event. The key idea is that even with practice, the reaction time can be reduced but not brought down to zero. The activity is part of a much larger afterschool program, but can be used by itself.