I found the information on the internet ages ago and never tried it until now.So i will share this instructable .
Step 1: Method
Basically the bottom of most cans have a parabolic shape that when polished will allow you to concentrate sunlight and make fire.You can use any polishing agent,I used small grit sandpaper and toothpaste.Work is circular motions and try not to press to hard thus changing the shape of the bottom of the can.You have to carry on until the bottom looks like a mirror and you can see yourself in it.Then use toothpaste or any suitable polishing agent.
Step 2: Burn baby burn
Once you are happy with your handy work and the bottom is nice and shiny its time to test.take a small piece of paper or a twig and use it just as you would a magnifying glass.