Open contribution systems such as Wikipedia and Linux have been extraordinarily successful at eliciting contributions from many volunteers, but other projects struggle. While research has examined general motivations for contribution, we know little about what triggers contribution at a specific time. In this paper we mine the history of Wikipedia to understand the contribution process in the context of the many competing demands on users’ attention at their computers. In particular, we examine the influence of the visible work of others on the timing and amount of contributions to Wikipedia. Using two different statistical models we show converging evidence of a substantial influence of others’ visible work on triggering contribution.