The Sun's Heat...

Today, I was remembering about the time when I went to one of my friends house and her brother, who wears glasses, used the glasses to make a fire just with a leaf and his glasses. So I found a magnifying glass, put a piece of rock down, put some dry grass and a paper towel on it, and I held the magnifying glass in front of the sun, which made a tiny dot right on the stuff, which started it on fire. If you are going camping and you forgot a box of matches but did bring a magnifying glass, this is an easy way to start a fire.

Here's a good explanation of why it works, from a website called campfiredude.com.

"When you look through a magnifying glass, an object appears larger because an area 1/32 inch in size at the focal point is spread across an area 2 or more inches at the lens, making something look 60x its real size. By having the sun's rays hit the lens all across its face, each of those rays will be redirected to the focal point. Instead of making something look larger, all the light and heat from the 2 inch surface are will be condensed into 1/32 inch area. making it 60x as hot. If the sun's rays are normally adding an extra 15 degrees to the surfaces they hit, 60x that would easily be enough to start tinder burning. "