Thermal Mass

http://www.motherearthnews.com/green-homes/thermal-mass.aspx

If your car is not in the shade on a sunny day (even in winter), it warms up quickly. That's the greenhouse effect.

If the windows in your house are in the right places, your house will warm up in winter, too. But how do you retain that heat? Thermal mass. If you're not already familiar with the concept, BuildingGreen.com has a good primer for you. Click here to read it.