The magic eraser

It was the best ten pounds Campbell had ever spent. All he had to do was wait for it to arrival in the mail. Then on one perfectly sunny day when everything was going perfectly-his toast nicely browned not burnt, his orange juice devoid of floaty bits, his mother not nagging at him-there was a knock at the door.
"Mail," Campbell beamed and leapt off his chair in the kitchen and skidded down the polished floor of the hallway to the front door.
There, waiting for him, in the hands of Martin the Postman, was Campbell's long awaited purchase.
Before Martin had even handed the rest of the mail to Campbell's mother, Campbell had torn open his package. He dropped the jiffy bag on the floor of the hallway and dashed upstairs to his bedroom. Nobody was going to get a look at it before he had.
Campbell read the front of the box.
"Magic Eraser. Mystify your teachers. Just wipe over blackboards and watch," he said and sniggered to himself. "I'm going to have such fun today."
All through the bus journey to school Campbell sat with a smug grin on his face. His friends were desperate to know what new trick Campbell had planned, but Campbell kept his mouth shut.
"Come on tell us," they pleaded. "We promise we won't tell."
Campbell never said a word. He simply smiled.
When the bus pulled up outside the school Campbell was the first to leave. He ran through the gate, across the school yard and into the school itself before any of his friends even knew where he'd gone.
He snuck into his class, taking care to check that no stray teachers were wandering the corridors, and clicked the door shut behind him. He dug into his bag, pulled out his Magic Eraser and swept it across Miss Carter's blackboard. He just finished when Miss Carter stepped into the room.
"Campbell, what are you doing in here?" she asked.
"Just cleaning the blackboard, Miss Carter," Campbell replied. It was the truth, of sorts.
He grabbed his bag and went over to his desk just as his classmates trundled into the class.
Proudly he sat exchanging knowing looks with his friends who he could see were burning to know what Campbell was up to.
"Right class," said Miss Carter. "We're going to start with fractions today."
Everyone groaned, except Campbell. And when Miss Carter started writing on the blackboard, his friends realised why.
Everything that Miss Carter wrote on the blackboard disappeared as quickly as it was written.
The class started to snicker behind their hands as Miss Carter stared bewilderedly at her chalk. She tried to write on the board again but as before the numbers disappeared before she had finished.
Again and again she tried scribbling faster and faster but nothing remained on the board. It was as black as the moment Campbell wiped his magic eraser over it.
Campbell sat back with his arms crossed and looked across the classroom. His teacher was flustered to the point that strands of her hair hung, bedraggled, around her face. His classmates were doubled over with laughter-some laying on the floor clutching at their stomachs, red-faced, others bent over their desks and chairs, clapping, cheering and pointing at the teacher.
"I wonder what I can do tomorrow," Campbell thought.