About once in every 30 days, Google recalculates the page rank of each web pages, and re-indexes the SERP (Search Engine Result Pages) for all keywords. The Google Dance is the time period between the start and end of the re-indexing. During Google Dance, the search results are different on www.google.com, www2.google.com, and www3.google.com. The www2 and www3 are known to be Google's test engines. The dance normally takes about 4 days.

Even when no Google Dance is in effect, SERPs may be different for same keywords at different time of the day. This may be due to hitting different Google servers between searches. Search results may be different on different Google servers, and geographic location and load balancing potentially makes users land on different servers.