Pop bottle Water Filter.

sometimes you must know how to do with what is on hand. If you don't have a chance to add a lifesaver water bottle to your 72 hour kit ( it can be purchased at Amazon.com ) (http://www.amazon.com/Lifesaver-Bottle-4000-Ultra-Filtration/dp/B001EHF99A/ref=sr_1_1?ie=UTF8&s=sporting-goods&qid=1309808137&sr=1-1)

http://www.ted.com/talks/view/lang/eng//id/613

Then to make one you take out a sharp object hopefully you have a multi tool in your kit. on most multi tools there is an awl this is used to poke a hole in the top of the cap then cut the bottom of the bottle out.
take a wedge of cloth and put that in the cap. Now turning the bottle upside
alternate each level. If you can use a piece of cloth between each level or cotton
1 Activated Charcoal. This may not be readily available.If you have access to an old can pop or beer can you can make your own charcoal by putting pieces of wood in it and placing the can in the fire pit.
2 coarse Sand and Fine sand separated.
3 Coarse Gravel and fine Gravel separated
4. Cotton or Cloth you can get by with pebbles