The Boy learned to make caramel at Thanksgiving (which seems so long ago). Last night he asked me why the sugar melts so suddenly, not gradually, but collapsing into a puddle all at once. "Because it is a pure solid. One test of the purity of a material is how sharp the melting point is." I have a vivid recollection of a sample of something I synthesized tamped into a narrow glass straw, its crystals suddenly losing their structure and collapsing into a liquid.
I'm not pure by any means, but I think I may have reached the melting point tonight, and it was quite sharp. I've been plugging away all week at a list of "things that must be accomplished" before weeks' end. Minutes ago I put the last touches on the exam that has to be ready tomorrow morning and just about collapsed into a professorial puddle.
One more task for the night and I really can collapse into bed.