I've been given a challenge of designing a cheap and efficient container that allows light in to heat something, but doesn't allow the heat out. The cheaper, the better. I thought of several ways, but one that stuck with me was a double paned window. That allows light in but heat out! But how to make a cheap container that acts as a double paned window? Well, air isn't exactly a great conductor, it's a decent insulator. So, my thought was this. Take two glass jars of about the same height but different widths. THe smaller width should have a SLIGHTLY smaller height than the other one. Put some adhesive on the bottom of the smaller one and stick it into the bigger one. Then, put the lid on the bigger one! That way, light comes in, heats whatever's in the smaller one, but the heat doesn't escape? DOes that sound like a good design?