Time is like wax dripping from a candle flame; in the moment, it is molten and falling, with the capability to transform into any shape. Then the moment passes, and the wax hits the tabletop and solidifies into the shape it will always be. It becomes the past; a solid, single record of what happened; still holding in its curves and contours the potential of the shape it could have held. When we look back on that bit of wax and are deeply pleased with the shape it has become, that is nostalgia.