Significance Tests of fluid intelligence are important for their broad association with effective cognition and lifetime achievement. An enduring question concerns basic cognitive mechanisms measured in such tests. Fluid intelligence is usually measured with complex problem-solving tasks, and in such tests, we suggest that the core limit is one of cognitive segmentation, or managing complex activities by selective attention to separate, simpler parts. Here we modify traditional fluid intelligence problems to test this hypothesis and to minimize the roles of working memory capacity and mental speed. The findings suggest a cognitive interpretation for what it is that fluid intelligence tests measure, based on dynamic attentional control functions of frontal and parietal cortex.