Do you carry a bandana while hiking? I always do! I find it to be one of the cheapest (they can be less than $2.00!) items that has a lot of purposes for me while hiking or camping. Here's just some ways I use it!
Filter Water: You can use a bandana as a water sifter if a silty puddle is your only water source. We don't often think about cloth as having holes, but the tiny little holes between a bandana's fibers can make a great way to filter water.
Breathing Mask: If you're near a forest fire for some reason, it can be a really basic breathing mask. Just tie it tightly around your face! It can also keep your face warm.
Makeshift Bag: If you tie it right, you can make a makeshift bag or ice bag out of it.
Trail Marker: Because bandanas are so easy to see and to tie, they're great to mark a trail for those who are following you, or in case you yourself get lost. Just make sure to tie that bandana tight, if it's important to you or your fellow hikers. If you buy the classic red bandana (or another bright color), it makes a simple emergency flag.
Makeshift Gauze: You can also use one for bandaging and splinting, because cotton is absorbent and heartier than gauze.