A supplementary lesson in physics

       

It is cold, and a little bit of snow has fallen onto the terrace table. – Only snow? No. For some reason a few black particles have dropped off the sky during New Year's Eve. Now the next day the sun shines bright, but still it is too cold to melt the snow. The white snow reflects most of the light. Only the black particles convert light into heat and melt the snow in their proximity.