Google sends out robots (spiders) to crawl the web and archive every website it finds. These archived websites will be presented in the next update of the Google index (database). The update process is initiated every 30 days or so, although on some occasions there have been more than 30 days between updates. During the update, which takes several days, the 3 Google servers display different results. Whilst the results vary from server to server, they are said to be "dancing" - Google Dance