A photoresistor is a variable resistor, a lot like the potentiometer that controls the volume of your speaker. The difference is that a photoresistor's resistance is controlled by the amount of light that hits it's surface. Typically more light causes less resistance. These are often used in Robotics projects to have a robot that reacts somehow to light. For instance, they can be used in combination with LEDs to create line-following robots. They can also be used in "Robot Tag" projects in which one robot shines a light on another robot to cause an effect, such as making the target stop moving, or adding to a hit-counter of some sort. The use of photoresistors as sensors in such applications requires knowledge of voltage dividers. Increasing or decreasing the output voltage with increasing light intensity depends upon whether the photoresistor is used for or , respectively.