If you ever find yourself in the junlge or wilderness and are trying to survive from a group of dangerous animals, hunters, or aliens (ala Predator), then you're going to need a weapon. Why not make a crossbow?
This next tutorial will show you what materials you will need to make an improvised crossbow from scratch. Tent poles, tape, some wood, and other stuff were all used in the creation of this weapon. So check it out and good luck.
Oh, please do not use this to harm anyone unless you find yourself in a dire situation.