"...there is little value in learning an opening which you will be given the opportunity to use only occasionally. Never box with boxers or grapple with wrestlers. Karate side kicks are best. What is the use of learning the 'right' moves if the opportunity for their use seldom presents itself?" Richard Wincor in Baroque Chess Openings.
1. "e4" is a common opening which is usually followed by "e5".
2. "f4", however, is a karate side kick that opens up a world of possibilities.