Nearly everyone knows (or should know) that when your engine "pings" under accelleration, that's bad. Many gearheads can tell you in vague terms why this is bad, but few (myself included) know exactly why it happens. But this site and this site do a bang-up (as it were) job of explaining the how's and why's of internal engine knock. The first one even has pictures of combustion as it ocurrs. Neat!