A Ravenclaw student answered instinctively:
"The reed pipe would burst instantly."
"Exactly right." Douglas snapped his fingers.
"A weak, untrained body is that reed pipe."
"If you force yourself to cast a Nonverbal Spell beyond what you can handle, your magic will run out of control inside you and backfire."
"At best, your magic will be depleted. At worst, your nerves will be burned, and your magical core could even suffer permanent damage."
Those words made the classroom air turn several degrees colder.
Only then did Douglas casually add:
"Of course, you needn't worry so much. For any of those accidents to happen, you have to be able to cast a Nonverbal Spell first."
The students gave embarrassed smiles.
Douglas surveyed the room.
"This is also why the school began making physical training compulsory last school year."
As he finished, a line appeared beneath the day's lesson topic, which was still hanging in midair.
[Intent strength = magical output × concentration]
"The body is the hardware, and the will is the software."
"You cannot do without either."
The theoretical explanation was over.
"Now."
"Copy my movements."
Douglas stood perfectly straight, his center of gravity steady and his shoulders relaxed.
"Feel your bodies, from the support beneath your feet to the stability of your core, then the extension of your arms."
"A powerful wizard must first be able to control their own body."
"Now, bend your wrists slightly, hold your wands level with your chests, and point the tips forward."
"Close your eyes. Don't think about anything."
"Just repeat one term in your mind."
"Shield Charm, Shield Charm, Shield Charm..."
Douglas thrust the wand in his hand sharply forward.
There was no spoken incantation, no complicated gesture.
Only a thick, sky-blue shield of light that formed instantly before him.