Teachers have been building mystery circuits or so-called "black box circuits" to use as a demonstration with their students for years. This paper presents an easy way to make simple mystery circuits using inexpensive light fixtures (see Fig. 1) available at almost any home improvement store. In a black box circuit, only the lightbulbs are visible and the wiring in these circuits is hidden from the students. The students are then presented with the challenge of deducing the nature of the electrical connections hidden by the black box. The students may remove one or more lightbulbs from the circuit to watch how the brightness of the other bulbs changes and, from this, deduce which bulbs are connected in series and which are connected in parallel.