Seeing someone make a fire out of the lenses of some glasses might seem like something out of a movie, but it can be done in the right conditions. Likewise, it's possible to craft a makeshift knife using nothing more complex than a piece of wood and a rock. The rock can be used to hit the wood with the rock repeatedly until it breaks. Then the rock can be used to sharpen the wood into a serviceable knife. Of course the best way to be prepared in the wilderness is to have a survival kit on hand just in case the worst happens.