“How do we do this one?” she shouted in exasperation from the middle of the room.
“Which one?” he asked without looking up as he finished scribbling some notes.
They were studying for a test. “The hardest one of the year,” he had warned them. This group was working hard (in contrast to the table in the corner).
“Problem 3,” she said as he walked up.
“Ok, watch,” he said. He went to the board where he had discussed that problem minutes earlier with a different group.
“We have a hole at x=3, right?”
“Right…” she said tentatively.
He reached up and erased some of the previous work and began explaining the function that remained.
“If you…”
“Wait wait wait,” she shouted. “I got it!”
And she turned and started explaining it to the others at her table.
That qualifies as a good day.