Snape had found an empty classroom in advance and led her inside.
"Although the Patronus Charm is an advanced spell, its incantation and casting method are actually very simple. Yet many students still cannot use it properly by the time they graduate."
Audrey asked in confusion, "Professor, that's exactly what puzzles me. Edward told me that the incantation for the Patronus Charm is 'Expecto Patronum,' and that when casting it, you need to concentrate and do your utmost to recall something happy. But judging by that description, it shouldn't be very difficult."
Snape's expression was cold. "First, what constitutes something happy differs greatly from person to person. Some people might successfully cast the Patronus Charm simply because of the happiness of drinking a glass of milk at breakfast. Others could rack their brains for every happy moment in their lives without getting the slightest reaction from their wands. For others still, happiness and pain are actually connected. Their happiness is equivalent to their pain, and they need to find happiness in the cracks between that pain..."
He paused for a few seconds. "Human thought is strange and uncontrollable. When you desperately try not to think about something, you inevitably think about it. When you urgently try to think about something, all sorts of jumbled thoughts flood in at the same time, disrupting your train of thought."
"The happy memories required for the Patronus Charm must be pure, clean, and free of any impurities. That is one of the main reasons most people cannot learn it."
Understanding dawned on Audrey.
Snape continued, "And happy memories are only the prerequisite. The key is that you need to summon your emotions, draw enough magic power from that happiness, and use that magic power to cast the Patronus Charm."
Snape gently waved his wand.