What do we think about when our mind wanders? We asked 88 students to read an instructional text and watch a film (each 20 minutes) and report whenever they found themselves zoning out. Each time they did, we asked them to report their thoughts and what, if anything, triggered them. We then categorized these thoughts (1208 in total) based on their content, and found that in contrast with previous studies, only 17% involved prospection whereas 33% consisted of autobiographical and semantic memory retrieval. This discrepancy might be driven by the rich content of stimuli: 71% of autobiographical and semantic retrieval was explicitly triggered by the text or film, compared to 28% of prospection. Latent semantic analysis revealed that memories were more similar to their triggers than prospective thoughts, suggesting that a substantial proportion of mind wandering is driven by the content of our environment.