How does Extension Tubes work?

[dRebelXT]

Googled a few links but couldn't find out the principle of its funcitons.
Care to explain?

 

[zekai]

its goes between the lens and the camera body altering the effective min focusing distance. remember learning abt it in pri school science. (physic)

 

[RossChang]

too lazy to elaborate... read below -->

http://forums.dpreview.com/forums/read.asp?forum=1029&message=11599864

http://forums.dpreview.com/forums/read.asp?forum=1029&message=9700613

http://forums.dpreview.com/forums/read.asp?forum=1031&message=9238086