Wikipedia:
Knocking (also called
knock,
detonation,
spark knock,
pinging or
pinking) in spark-ignition [ame="http://en.wikipedia.org/wiki/Internal_combustion_engine"]Internal combustion engine - Wikipedia, the free encyclopedia[/ame] occurs when combustion of the
air/fuel mixture in the cylinder starts off correctly in response to ignition by the [ame="http://en.wikipedia.org/wiki/Spark_plug"]Spark plug - Wikipedia, the free encyclopedia[/ame], but one or more pockets of air/fuel mixture explode outside the envelope of the normal combustion front.
Just that simple. Argument over.