We examine 2,465,145 interactions from 534,507 users of Dogpile.com submitted 6 May 2005. We compare query reformulation patterns. We investigate the type of query modifications and query modification transitions within sessions. Searchers most often modified their query by changing query terms (nearly 23% of all query modifications). Searchers' queries undergoing modification typically transition from Web to image collections in content shifts (37% of all content transitions), and searchers typically implement assistance at the start of a session or when switching content collections (21% of all assistance usage). This research sheds light on the more complex aspects of Web searching involving query modifications