Pair programming is a strategy that grew out of industry and has shown promise for performance and retention in computer programming courses at universities and in industry. In this study, we examine whether pair programming is effective in K-12, what it is effective for, and how partners influence each other. We collected the data from 320 middle school students working alone or with a partner on a game programming task. Compared to working alone, pair programming was advantageous for computational thinking and for building programming knowledge, particularly among less experienced students. Students' initial computer experience and attitude toward collaboration influenced their partners. Those with more experience relative to their partner achieved greater computer confidence and more positive attitudes toward computing. When students had a more positive attitude toward collaboration (relative to their partner), their programming knowledge decreased. We discuss implications for pairing students and for future research.