On the board it says in gigantic letters:
Save — Do Not Erase!
a mathematical equation
surrounded by a huge protective square,
so important it must remain there,
smack in the middle of the board,
to be contemplated by a class
at least twice.
He looks at it knowing
he’ll need the entire board for his class,
lifts the eraser with a heavy heart,
hovers over the equation,
then lets the black felt leveler
come back to rest in the tray.
The equation survives,
will live another day for further scrutiny,
and what he has to write for his class today
he’ll fit in around the margins,
erasing liberally as he speaks on,
ideas that will not outlive the hour,
knowledge that will pass away by his own hand,
so ephemeral the nature
of all the unsaved words he has to say.