Caching is saving already-rendered HTML output and other calculated data for later use the first time it is needed. This saves time, because the next time the same data is needed it can be quickly retrieved instead of recalculated. Automatic caching systems also include mechanisms to delete cached calculations or mark them as no longer valid when the underlying data changes. To facilitate that, cached data has a <em>lifetime</em>, which is the maximum time before the data will be deleted from the cache (forcing recalculation).