They were learning a new spell. Sarah liked the thought of it. She knew scourigfy so she did it first.

She faced her desk and said "Scourgify". The desk looked much cleaner. Yet now she had to polish it too. That was what the new spell was for.
"Poliotus" she said pronouncing each sylable with care. "Poliotus" she repeated again and again. This was her way of getting her tongue aquainted with the incantation.

Then she practiced the wand movement. It was just a simply S which was easy.

"Poliotus" she said making an S over her desk. To her surprise it worked the first time around and her desk now shone!