Here's a technique you can use to start a fire for survival if you having nothing but a water bottle and a bright sunny day. If you like survival and fire-making techniques, this video is a must see! http://www.thekingofrandom.com Project inspired by: A co-worker mentioning her boyfriend once burned a hole in a pice of paper with a water bottle. On that idea, I practiced until I could prove it worked, and then until I felt like I'd mastered the technique. WARNING: Use of this video content is at your own risk. Music by Kevin Macleod (Stomp Dance, Movement Proposition, Chipper Doodle) http://incompetech.com/ Computer paper with black ink is the tinder of choice for practicing this technique because anyone should have access to it, and it's fairly consistent and reliable. The bottle needs to be filled with water, and ideally should have no air bubbles inside. By turning the bottle upside down and using the rounded convex part at the top, we can use a water bottle as a make-shift magnifying lens. The curved water refracts the sunlight, and give the ability to focus it. The closer the rounded lens is to resembling a liquid sphere, the better it will work. If bubbled are trapped in the bottle, they will distort the sunlight and weaken the intensity of the beam. This technique isn't easy, and takes a lot of practice to master, so don't wait until you're in a survival situation to try it.