A spark plug is a small device in gasoline engines that creates a tiny lightning bolt to ignite the fuel. Inside each cylinder of a car's engine, gasoline mixes with air, and at just the right moment, the spark plug fires an electric spark across a small gap. That spark ignites the fuel mixture, causing a controlled explosion that pushes the piston down and makes the engine run. Without spark plugs firing thousands of times per minute, your car wouldn't move.

Most car engines have one spark plug for each cylinder, usually four, six, or eight. Mechanics replace spark plugs when they wear out, because a failing spark plug can cause rough idling, poor fuel economy, or an engine that won't start at all.

People also use spark plug as a metaphor for someone who energizes a group or gets things moving. If your friend always comes up with ideas that get everyone excited about a project, you might call her the spark plug of your team. Like the device in an engine, a person described as a spark plug provides the initial burst of energy that makes everything else work.