A series is a circuit in which all of the  elements are arranged in a single path and the current passes through all of the components. The one that we designed included a foam board, battery pack, 2 switches, and 2 different LEDs. The red wire was hooked up to the start of the series and the black wire was at the end, completing the circuit. We had trouble figuring it out because it was difficult to loop the wire into the switch or get the LED to turn on.