Professor Maddox filled the blackboard the way she filled her students' minds: methodically, from left to right, one column of equations at a time. By the end of the lecture every cell was covered in chalk. She began erasing from the left, clearing one column per day for the next week's material. But the teaching assistant, trying to help, erased from the wrong end — wiping the rightmost column, the material they hadn't reviewed yet, while the columns she'd meant to clear still stood. She stared at the board: two clean columns on the edges, the untouched lecture still filling the middle. She started over.