"The power of two" is a Royal Institution (Ri) mathematics "master-class". It is a two-and-a half-hour interactive learning session, which, with varying degree of coverage and depth, has been run with students from Year 5 to Year 11, and for teachers. The master class focuses on an historical episode--the Josephus Problem--given a circle of "n" people where every alternate living person is killed in succession, where should one stand in order to be the last person alive, the survivor? Working in groups, through a mixture of pattern-recognition and problem solving, the challenge is accessible and provides an informal introduction to number systems, in particular, place value, binary representation, and the notion and importance of proof. This article is based on the experience of Jan Chodurek, who adapted the master-class to meet the needs of learners at Finchley Catholic High School.