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Caching is a great example of the ubiquitous time-space tradeoff in programming. You can save time by using space to store results. In the case of websites, the browser can save a copy of images, stylesheets, javascript or the entire page. The next time the user needs that resource (such as a script or logo that appears on every page), the browser doesn’t have to download it again. Fewer downloads means a faster, happier site.