One of the most fascinating properties of the brain is its ability to process identical sensory stimuli into distinct behaviours. This is the basis of learning and decision-making, and what enables animals to respond to their environment according to their ever-changing needs. What are the mechanisms that provide brains with this flexibility in function? To address this question, we focus on three important factors that influence the way sensory information is processed: previous experiences, mood or internal state, and biological sex. Our experimental system is the free-living round worm Caenorhabditis elegans, a genetic model organism with only 387 neurons (in males) and highly flexible behaviour. Because of its small size, amenability to experimentation and the conservation that exists in the way nervous systems work, C. elegans helps us understand at an unprecedented level of resolution how behaviour is produced by cellular and molecular reactions.