As a preparation for the International Mathematical Olympiad there is a common training week for the UK and the Hungarian squads. Recently I gave a talk about cross ratio there. This weapon is not one of the most frequently used mathematical techniques. When attacking a problem, we ask what are the signs that cross ratio might be of use in finding a solution? Are there typical situations which invite us to use it? In this paper we make some suggestions about answering such questions. The reader will find proofs for well-known theorems and solutions of various problems using cross ratio.