A stoplight is a set of colored lights mounted at an intersection to control when vehicles and pedestrians should stop and go. You've probably noticed them at busy street corners: the familiar red, yellow, and green lights stacked vertically in a box or hung from a wire overhead.

The system works through a simple color code. A red light means stop and wait. A green light means it's safe to go. A yellow light warns that the light is about to turn red, so drivers should prepare to stop if they can do so safely. The lights cycle through this pattern automatically, giving each direction of traffic its turn to move through the intersection.

Before stoplights were invented in the early 1900s, busy intersections were chaotic and dangerous. Traffic police had to stand in the middle of streets directing cars, which was exhausting and risky. The first electric traffic signal was installed in Cleveland, Ohio, in 1914. Today, stoplights make city streets much safer by creating an orderly system everyone understands.

Sometimes people use stoplight as a metaphor for any go-or-stop signal. A teacher might use a stoplight chart in class, where green means you're doing great, yellow means you need to refocus, and red means you need to stop and think about your choices.