How do you make a fire with a lens?

Take the lens into a place where the sun is shining. Hold the lens between the sunlight and the object you wish to burn. Move the lens nearer or further away until the lens throws a small usually pale blue circle on the object. Hold the lens steady until wisps of smoke appear. Whether the object catches fire will depend upon the nature of the object. Dry paper will burn but wet cloth will only steam until the material is dry.