When you're out hiking and need a small backpack for an emergency, a pair of pants can be quickly turned into a carrier with some extra cord or string, letting you carry your water bottle, lunch or small supplies.
Here is Ludvicka's Instructables guide:
Step 1: Cut the cord into three pieces. Two about 12 inches and one about 3 feet. Tie each pant leg at the bottom about three inches from the hem. Tie them pretty tight.
Step 2: Pull the two legs up and tie to the belt loop on either side of the zipper. Be sure to tie them tightly.
Step 3: Run the longer cord through the belt loops and use as a drawstring to close the top of your back pack. Fill with softer things on the side against your body and any other things towards the front or back of the pants. The pockets can hold your water bottle, gloves, cell phone, etc. The more pockets the pants have the better. Slip on your new daypack and off you go.
It's simple and you can get it done pretty quickly. Bonus points if they're rain trousers!
Pants Daypack/Backpack [Instructables]