This paper investigates the characteristics of 15-year-old girls who express an intention to study physics post-16. This paper unpacks issues around within-girl group differences and similarities between boys and girls in survey responses about physics. The analysis is based on the year 10 (age 15 years) responses of 5,034 students from 137 UK schools as learners of physics during the academic year 2008--2009. A comparison between boys and girls indicates the pervasiveness of gender issues, with boys more likely to respond positively towards physics-specific constructs than girls. The analysis also indicates that girls and boys who expressed intentions to participate in physics post-16 gave similar responses towards their physics teachers and physics lessons and had comparable physics extrinsic motivation. Girls (regardless of their intention to participate in physics) were less likely than boys to be encouraged to study physics post-16 by teachers, family and friends. Despite this, there were a subset of girls still intending to study physics post-16. The crucial differences between the girls who intended to study physics post-16 and those who did not is that girls who intend to study physics post-16 had higher physics extrinsic motivation, more positive perceptions of physics teachers and lessons, greater competitiveness and a tendency to be less extrovert. This strongly suggests that higher extrinsic motivation in physics could be the crucial underlying key that encourages a subset of girls (as well as boys) in wanting to pursue physics post-16.