Subjects submitted strategies for playing a public good game. The strategies interacted with each other in a computer simulation. They could be revised and resubmitted for a second and a third simulation round. The strategies' average contribution to the public good increased from one simulation to the next. The majority of the strategies submitted for the final simulation show the same pattern: In the first period, they signal their willingness to cooperate by contributing the entire endowment to the public good. Then, they reciprocate the others' average contribution in the previous period. In the final period(s), cooperation is given up, whatever the history of the game.