How do scientists make inferences for something they cannot directly observe? The Black Box approach seems ideal to help students understand how scientists work. Black Boxes are sealed units; their interior is not accessible. The effort to determine their possible content (internal structure) demands successive modifications in hypothesis, measurements, and observations, without opening the Box. There can be more than one suggested inference for its internal structure, compatible with all observations. This lesson asks students to identify Nature of Scientific Knowledge aspects using a Black Box problem-based activity on electric circuits, and was part of a greater project on the Nature of Science in an 11th-grade science class in Greece.