The testicles (balls) and the penis grow larger. The scrotum (the sack holding the testicles) may darken in color.
Sources:
Visit www.seksuelevoorlichting1991.nl (a verified historical project by the University of Groningen). There, you will find scanned lesson plans, teacher guides, and parental letters—every claim in this article cross-referenced with primary sources. The testicles (balls) and the penis grow larger