some of the posher ones among us have quick release rear sprockets, I have a Talon (brand name) one. Basically, they come in two pieces - the carrier which bolts onto the hub (the bit that spins round with the back wheel), and then the outer ring with the teeth on that the chain goes round.
This outer ring can be taken off and swapped with one of a different size to change the gear ratio (changes the number of times the back wheel spins in relation to the engine), and is held on by five little bolts. Take the bolts off and the sprocket ring will just fall off..... hence me being a bit serious about three of them coming out on their own on the motorway!
Would post a piccie of it but I'm so technically inept that I've yet to work out how to charge my camera battery!!
edited.... found some batteries, but now the tv won't change channel !!
Gold bit separates from the grey bit, and is held on by the 5 SMALLER bolts. Easier with photie I hope!
[Edited on 6-7-2004 by NBs996]