The crossword solution for the term 'REACTION CAUSES' is 'STIMULI'. STIMULI are external or environmental factors that can cause a reaction in an organism. These reactions can be physical, mental, or emotional, and can range from simple reflexes to complex behaviors. STIMULI can be anything from light and sound to temperature and pressure. They can also be social or psychological, such as words or images. STIMULI can be used to study behavior and can be used to help understand how organisms respond to their environment.