A Spark Plug is a component of a combustible engine commonly found in Cars and Trucks. It delivers a spark within the engine which ignite the gasoline which drives the engine.
While Joe and Vinny were near John Jack's car behind The Drunken Scotsman, they had a discussion on how to remove the spark plugs. Since they didn't know how to remove the spark plugs, Joe recommended dropping a Grenade under the hood.
External link[]
Spark plug on Wikipedia