To study effects of the gender-packaging of science, technology, engineering, and mathematics (STEM) toys, mother-child dyads (31 daughters; 30 sons; M = 5.2 years) were randomly assigned to play with a mechanical toy packaged for girls ("GoldieBlox") or boys ("BobbyBlox"). When familiarizing themselves with the toy to prepare for play, mothers given "BobbyBlox" built more with toy pieces than did mothers given "GoldieBlox." During dyadic play, mothers with sons built more; mothers with daughters read the toy's narrative instructions more. Children's independent play likewise varied with game packaging. Girls learned the mechanical belt-drive principle better from playing with "BobbyBlox"; boys learned the principle better from playing with "GoldieBlox." Implications for gender-schema theories, STEM interventions, and toy marketing are discussed.