Young children's everyday scientific thinking often occurs in the context of parent-child interactions. In a study of naturally occurring family conversation, parents were three times more likely to explain science to boys than to girls while using interactive science exhibits in a museum. This difference in explanation occurred despite the fact that parents were equally likely to talk to their male and female children about how to use the exhibits and about the evidence generated by the exhibits. The findings suggest that parents engaged in informal science activities with their children may be unintentionally contributing to a gender gap in children's scientific literacy well before children encounter formal science instruction in grade school.