Spark Plug
Not many people realize that when people talk about "pulling the plug," they're really talking about spark plugs.
Example
LOVED ONE: I think I'm going to pull the plug.
NP: That'll mean less horsepower in your car if you do so.
LOVED ONE: I know, but I think it's time to let go.
NP: I completely understand. (Short pause.) Are you sure you don't want to talk about anything else other than cars?
Other Meanings
A spark plug can be a term used to refer to a highly-enthusiastic medical student like Karen Weaselsnout-Jones who can't wait to talk your ear off about the Krebs Cycle, or interns like Timothy McDuffy who can't wait to cause their first pneumothorax.[1][2]
Related Medical Mechanics
References