What packs up smaller than a pin head then gets like, 20,000 times bigger to make a 20 litre daypack that is extremely light and tough - don't know, well then let me tell you young pedawan - It's called the Sea to Summit ultrasil daypack and it kicks your and my ass.
Actually it packs up to about the size of a rabbits fist (you know, when they fight and stuff) and really is 20 litres when expanded - perfect for us fellas when it's too warm for a top or jacket with pockets and you don't want to be dragging that new gucci handbag around with you - just stick this in your pocket and forget about it, that is until you need it then kapow!, pull it out and wave it around cos you just saved the day captain bagface. Oh yeah and girls can use it too.
Below - the best bag ever made? - possibly.