How do these fans work?

http://www.lehmans.com/shopping/product/detailmain.jsp?itemID=306&itemType=PRODUCT&RS=1&keyword=fan

"Reduce hot and cold spots in your room. Fan powered by the heat of your 
stove circulates air away from stove and throughout the room. 
Ingeniously designed fan converts thermal energy into electrical energy. 
Blade automatically increases speed as the stove gets hotter and is 
specially designed to gently move a broad section of air. Unobtrusive 
and virtually silent. Will amaze and intrigue guests - a great 
conversation starter! No maintenance required. Aluminum body and 
anodized brass blades, one-year warranty, made in Canada."

Neat idea, but way too expensive for my budget!  Would like to try to 
make one though... :)

Aaron
-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist