Search engines use invisible “bots” or “spiders,” which are really programs or automated scripts, that browse (or “crawl”) the World Wide Web in a methodical, automated manner. Search engines use spidering as a means of providing up-to-date data. This type of technology is necessary because the rate at which people create new Internet documents greatly exceeds any manual indexing capacity. In fact, an estimated 20 billion web pages exist, and search engines have crawled about half of them.