But if the unburned charge ahead of that expanding flame front is already unusually hot as a result of abnormally early spark timing, an excessive compression ratio, or an overheated engine, chemical changes in the unburned mixture move faster than normal, generating an increasing population of highly reactive molecular fragments, the most active of which is OH-, the dreaded hydroxyl radical. If this population reaches a certain density, bits of the remaining unburned mixture out near the cylinder wall can auto ignite—go off by themselves—before the flame front reaches them. In their chemically-altered state, these bits mixture burn, not at the usual slow 50 to 200 feet per second, but at the local speed of sound, which may be as high as 3,000 feet per second. This creates shock waves, zones a few molecules thick, across which there is a very steep pressure gradient. When these pressure waves hit internal engine surfaces, we hear engine knock. In a taxi engine, running cheap gas and lugging, it sounds like knocking two stones together under water. In a racing two-stroke, some riders say they hear a squeak.