"You gotta hold the wand like this-" the strange child demonstrated. "-and then you have to think of your happiest memory. Concentrate on it, let it overwhelm you, and then you have to say 'expecto patronum.'"
The child handed the stick to the man in Converse trainers.
Looking incredulously at the scarred boy, the man attempted the 'spell.'
"Tch. You have to think of you happiest memory of all time. You know, when you kissed the cutest girl-" the man shot him a dark look. "-or boy-" he shot a darker look. "-sorry. I dunno, your favorite gift. It's just gotta be your happiest memory. A dance or something." The lad threw up his arms in desperation.
The man smiled faintly. He whispered, "Expecto patronum."
A silver wolf bounded once from the wand and then began to pad slowly around the room.
"A wolf?" asked the boy curiously. "Really? One's patronus reflects their personality. I would've pegged you for a lion, or maybe a hawk."
As the wolf turned to look at her caster, the man quietly murmured too soft to hear: "Not just any wolf, the Bad Wolf."
The wolf gathered her strength, and in three swift strides she had charged her caster, striking his chest and dissolving, changing into a silvery smoke that drifted upwards to caress his face.
When the patronus completely vanished, and the man opened his eyes, the expression of joy and remembrance leaving his face, the lad stood there, awestruck and jaw dropped. He clanked his teeth together and tried to stutter something of an explanation. But the man hushed him with a wave of his hand, returning the wand to its owner.
"I don't get it," said the boy, breaking the silence. "You're nothing like a wolf."
"And your Potions Master is nothing like a doe," retorted the man, before he vanished.