When you try to see what's going on inside a black box, it helps if there is a window. In this talk, I will illustrate how eye-tracking allows us to peek into the inner workings of the human mind. I will use two case studies to make my point. In study 1, I will use eye-tracking evidence to demonstrate that people spontaneously simulate counterfactual possibilities when making causal judgments. In study 2, eye-tracking evidence reveals how people engage in mental simulation when trying to infer what happened in the past based on visual and auditory evidence. Together these studies show that people build rich mental models of the world and that much of human thought can be understood as cognitive operations on these mental models.