Q: I teach high school science, and my students are curious about compact fluorescent light bulbs. I hope you can help me out with these two questions about the basic functions of these bulbs. Why are they spiral? What is the function of the base? Thanks in advance, Rachael! A: Im glad to help. You can start by directing your students attention to the familiar overhead tubes that most likely light your classroom. Im sure theyll be able to identify these as fluorescent lights. You can explain that the more light we want from a fluorescent light, the greater a tubes surface area needs to be. If your class then takes a look at a compact fluorescent light (CFL), theyll see that this, too, is a tube. CFLs have simply been shaped into a spiral so that theyll take up the same amount of space as a traditional Edison bulb without losing the surface area needed to produce good quality, bright light. Thus the term compact fluorescent. As for the base of the CFL, its called a ballast. This is an electronic device that provides the voltage necessary to turn on the light and then regulates the voltage to allow the CFL to operate with the highest efficiency. Ballasts serve the same purpose in those overhead fluorescent lights in your classroom, but theyre part of the fixture, not of the tube. If your students are interested in learning more about energy efficiency, you can arrange for a speaker from Vermont Energy Education Program to conduct a hands-on workshop in your classroom. Visit www.veep.org. Q: I have good quality, double-glass windows that have been in the house since before I bought it. A few of them have a patch of what looks like moisture between the two panes that obscures part of the view. Is this a sign that the windows no longer are energy-efficient and should be replaced? A: That foggy patch indicates that the airtight seal between the panes has weakened and allowed moisture between the two layers of glass. This doesnt necessarily mean that the energy-saving performance of the windows has been significantly reduced. Even if this had been a sign of lowered efficiency, replacement wouldnt be recommended. Replacing a window for energy-saving reasons, alone, usually is not cost-effective. Dont get me wrong; Im all for energy-efficient windows. But theyre an investment, and one that is unlikely to pay for the full replacement cost in energy savings. However, if youve got to replace a window that is non-functional or if you need one for a new space, by all means get the best, energy-efficient window you can afford. At that point, an energy-efficient window becomes cost-effective, because your energy savings will pay for the difference between the price of a basic window (an amount youre going to pay, regardless of what window you pick) and the higher price of an energy-efficient window. If buying new windows, look for a U value of .32 or lower to keep heat in. And, unless youre installing east- or west-facing windows, which are most subject to overheating in the summer, look for a solar heat gain coefficient of at least 0.4 to allow in more heat from the sun.