Many genetics units do not explicitly address the concept of randomness. This simple, animated simulation can be used as a formative assessment activity to determine what students understand about this idea. The simulation allows users to enter in a number of car trips into a traffic circle and then see how many trips are safe and how many result in crashes. The app tallies and displays the results. Repeating this simulation can be the basis for classroom discussion about the idea that randomness means that the next event has no relationship to the event that preceded it and will have no effect on what happens afterward. It is a counter-intuitive concept that each event is independent even though there is an overall distribution or pattern.