Hold a concave mirror in your hand (or place it in a stand). Direct Its reflecting surface towards the sun. Direct the light reflected by the mirror onto a sheet of paper held not very far from the mirror. Move the sheet of paper back and forth gradually until you find a bright, sharp spot of light on the paper. Position the mirror and the paper at the same location for few moments. What do you observe? Why does the paper catches fire?

Explanation

The concave mirror focus the sunlight at one particular point. At that particular point, amount of sunlight is converged and heat is trapped at that point. This is because, it burn the paper on which the image is focused.