In 1948, Karplus married Elizabeth Frazier, whom he had met at an international folk dance group he organized while at Harvard. They had seven children born between 1950 and 1962. When the oldest child, Beverly, was 8, Karplus accepted her invitation to present a science lesson on electricity to her third-grade class, using the Wimshurst machine he had inherited from his grandfather. Unfortunately, while the children enjoyed the demonstration, the lesson was a conceptual disaster. This stimulated Karplus to think about how to teach science better, and as the other children entered school, he continued to visit their classes on a "show and tell" basis with various science experiments or demonstrations. Conversing with his children and their classmates, he became increasingly interested in children's learning, reasoning, and science concept development.