Some series make minor changes to the title sequence of each episode, such as superimposing a different episode title on each one. Others make minor alterations to the content of the sequence itself, to keep them from being completely repetitive each episode and to reward attentive viewers. For example, "The Rockford Files" would feature a different message left on the title character's answering machine, and "The Simpsons" features several unique elements in the title sequence of each episode (e.g. the couch gag).