A stimulus is something that causes a reaction or response. In science, when you touch something hot, the heat is the stimulus that makes you jerk your hand back. When your doctor taps your knee with a small hammer and your leg kicks forward, that tap is the stimulus. The response happens automatically because your body reacts to what it senses.

Scientists use this word to describe anything that triggers a reaction. A loud noise is a stimulus that might make you jump. The smell of cookies baking is a stimulus that makes your mouth water. Even light is a stimulus: when you walk from a dark room into bright sunshine, your pupils shrink in response to that stimulus.

In psychology, a stimulus can be more complex. The bell in Pavlov's famous experiment became a stimulus that made dogs salivate because they learned to associate it with food. Teachers might use different stimuli to help students learn, such as colorful posters, hands-on activities, or interesting questions.

The plural of stimulus is stimuli. When researchers study how animals or people respond to their environment, they're often studying how different stimuli affect behavior. Understanding stimuli helps scientists figure out how our senses work and how we interact with the world around us.