Darwin's theory of evolution is explicitly competitive, yet co-operation between individuals is a common phenomenon. The Prisoner's Dilemma model is central to the teaching of the evolution of co-operation. The best-known explorations of the Prisoner's Dilemma are the tournaments run by Robert Axelrod in the 1980s. Aimed at students of biological or behavioural science (post-compulsory education), this paper proposes a simplified, non-computer-based version of Axelrod's Tournament which allows students to explore the Prisoner's Dilemma using different behavioural strategies. In this classroom exercise, students become the players in a repeated Prisoner's Dilemma situation. They investigate the outcomes of different strategies of play using a simple scoring system. After each generation of play, students discuss the evolutionary success of the strategies--strategies that score poorly are discarded and replaced with those that score well. Four generations of play are suggested, after which "Tit for Tat" will have become the dominant strategy in the population, as it did during Axelrod's original tournaments. Students gain a basic understanding of the evolution of co-operation, and the associated written report encourages them to investigate the process in more depth.