In addition to the hard drive, iPod has a 32 MB memory cache. The cache is made up of solid state memory, meaning that it has no mechanical or moving parts, so it is not affected by movement of the device. iPod skip protection works by preloading up to 20 minutes of music into the cache at a time. iPod plays music from the memory cache rather than the hard drive, so even rigorous activities won't cause music to skip.