It's called Passive Solar Heating. I use 2ltr soda bottles painted black and filled with water. I keep my extra, extra tropicals on top of them as I've found it will keep the soil about 5 degrees warming than the rest of the plants not on them.
They collect heat in the day time and release it at night. Even in direct sun in the Winter they don't get as warm/hot as you think they would, but the do work for me!
