Heat from Sunlight
Ever used a magnifying glass to ignite paper? You probably know what lenses can do with a little sunlight. Using a Fresnel lens (a type created in the early 1800s for use in lighthouses) you can focus sunlight into a pool of water, raising or lowering the lens to adjust the rate of heating. As the liquid heats, a thermal-imaging camera shows the pool's convection patterns and temperature.