Smooth Black Stone

Gather three smooth black stones, as like to each other as possible.

Outdoors, in the sunlight, place the stones on a piece of cardboard. Leave one stone uncovered. Cover one stone with an upside-down jar or clear drinking glass.
 

For the third stone construct a simple test house with a 1-litre (1 qt.) and a 2-litre (2 qt.) milk or juice containers. You will also need Kleenex (or shredded paper towel or dry sawdust or cotton batten) as insulation, some clear plastic wrap, and an elastic band. Cut the top from each carton, about 10 cm (~ 4") up on from the bottom on the large one, and about 8 cm (~ 3") up on the smaller one.

Lightly scrumple up some kleenexes and place a few in the bottom of the bigger box; then place the little box inside the bigger box, and fill in the spaces between the walls with more scrumpled kleenexes (or other insulating material). Cut a piece of plastic wrap to fit over the opening and down the sides a bit. Put your third stone into the “house” and cover the opening with the plastic wrap, using the elastic band to hold it.
 

With your back to the Sun, decide how to orient your “house” so that sunlight shines into the box and onto the stone. (You may want to prop up the “house” so that its window is facing directly toward the Sun).

Make sure all three stones are in sunlight. A slight breeze may be blowing. Consider that the glass and the “house” protect those two stones from the cooling breezes. Wait about 1/2 hour.

Return. Check the temperature of the exposed stone, by picking it up in your hand and feeling it, and compare that temperature to the temperature of the stone that was covered by glass alone. Then open up the "house" unit, and feel the warmth of the third stone.

Here, solar energy warms the rocks. The warmed air in the glass and the room is protected and does not blow away. Insulation in the walls of the test house keep heat from radiating away quickly. Meanwhile, more and more solar energy is coming in through the window.
You might want to experiment with different insulating materials, and double glazing (by covering the interior box with a plastic wrap first), and with reflectors. Speaking of reflectors, what might be one way to keep heat in at night?