Magnification

You can use the lens from a magnifying glass or binoculars to start a fire. While you're unlikely to have a magnifying glass on hand in the wilderness, any concave piece of glass will do. If you wear glasses, you can even use them as long as the lenses are clear glass.

This method only works when the sun is shining. You have to use the lens to focus sunlight on your tender until it heats up and starts to burn.

Here's one that's a little tough to believe, but actually does work. You can form a lens out of a chunk of clear ice. Cut an area out into the shape of a lens and polish it smooth by rubbing it between your hands. Use it in the same manner you would a glass lens.

You can fill a condom with water and use it if you have one handy. Fill it, tie it off and squeeze it in various areas until you get a good focal point of light. You're going to have to hold it a couple inches from your tinder to get it to light up.