I just stumbled across this site which describes how to make a fire using a coke can and a chocolate bar. Basically you polish the bottom of the coke can using chocolate, which apparently acts as an abrasive, to make a parabolic mirror, and use it in a similar way that you would use a magnifying lens. I haven't tried it yet, but it's a neat idea, and I have no reason to doubt it would work. It's always good to learn new ways of making fire from improvised tools.