Solar panels for heat are usually made from a box with a clear window on top. Pipes run through the box. The pipes and the box are usually painted black because black absorbs more heat than other colors. The pipes are filled with a heat transfer fluid such as water or oil. A pump circulates the fluid, which heats up as it is exposed to the sun. When the hot fluid leaves the panel, it goes into a heat exchanger, which transfers the heat into water or air. After the now cool fluid leaves the heat exchanger, it is pumped back up into the panel to collect heat again.