learn to use a microscope to examine natural objects, and build their own collections of natural objects to be displayed in a special room of the museum. In addition to emphasis on allowing interaction with objects, Gallup also encouraged learning through play. She believed learning at the Brooklyn Children's Museum should be "pure fun", and to this end developed nature clubs, held field trips, brought live animals into the museum, and hired gallery instructors to lead children in classification games about animals, shells, and minerals. Other children's museums of the early twentieth century used similar techniques that emphasized learning through experience. Children's museums often emphasize experiential learning through museum interactives, sometimes leading them to have very few or no physical collection items. The Brooklyn Children's Museum and other early children's museums grew out of the tradition of natural history museums, object-centered institutions. Over the course of the twentieth century, the children's museums slowly began to discard their objects in favor of more interactive exhibits. While children's museums are a more extreme case, it is important to note that during the twentieth century, more and more museums have elected to display fewer objects and offer more interpretation than museums of the nineteenth century. Some scholars argue that objects, while once critical to the definition of a museum, are no longer considered vital to many institutions because they are no longer necessary to fulfill the roles we expect museums to serve as museums focus more on programs, education, and their visitors. After the Brooklyn Children's Museum opened in 1899, other American museums followed suit by opening small children's sections of their institutions designed with children in mind and equipped with interactive activities, such as the Smithsonian's children's room opened in 1901. The Brooklyn Children's Museum also inspired other children's museums either housed separately or even developed completely independently of parent museums, like the Boston Children's Museum (1913), The Children's Museum of Detroit Public Schools (1915), and the Children's Museum of Indianapolis (1925). The number of children's museums in the United States continued to grow over the course of the twentieth century, with over 40 museums opened by the 1960s and more than 70 children's museums opened to the public between 1990 and 1997. The next earliest children's museums were: Boston Children's Museum (1913) The Detroit Children's Museum (1917) The Children's Museum of Indianapolis (1925) – according to the ACM, this is the