"To perform this spell, simply say the incantation and wave your wand over the glass you wish to clean. For example..." Ivy got out her wand and drew a small line in the air in front of the glass while saying "Vitrumonia."

Forrest didn't practise Vitrumonia wandlessly again as he'd made sure before that his pronunciation was perfect. So, he simply moved towards the windows and found himself a spot among people. He was careful to choose somewhere less dirty than the rest of the windows because it was easier to clean and got his wand out to perform the spell.

"Vitrumonia."

... ... ...

Nothing, nothing and nothing.

He collapsed on a nearby desk, not really caring whether it was clean or dirty, and sighed to himself. He wanted to succeed in spells at the first try already.