Will shook his head. "Not really. I only know a little about the Memory Charm."
"Then why don't you tell me what you know, Mr. Walking Library of Hogwarts?" Dumbledore grabbed a handful of Cockroach Clusters and stuffed them into his mouth.
Will hoped a real cockroach had gotten mixed in. After a moment's thought, he said, "The Memory Charm was invented by the witch Nymone Radford in the fifteenth century. She went on to become the Ministry of Magic's first Obliviator.
"Since then, the Ministry of Magic has often used it to erase Muggles' memories of magic and keep them from discovering the Wizarding World.
"The departments that use it most are the Department of Magical Accidents and Catastrophes, the Department for the Regulation and Control of Magical Creatures, and the Misuse of Muggle Artefacts Office under the Department of Magical Law Enforcement. Muggles are always stumbling across Fantastic Beasts or enchanted objects.
"Large-scale incidents and magical disasters can also expose the Wizarding World. When that happens, wizards cast Memory Charms on great numbers of Muggles. Quite a few are left with lifelong memory problems because the wizards casting the spells aren't very good at it.
"And when the Wizarding World is exposed on a large scale, Memory Charms are often inadequate. Wizards usually can't be sure they've cast one on every Muggle who saw what happened.
"To conceal the existence of fire dragons, the Ministry of Magic has to keep casting spells on Muggles who spot wild ones. More serious breaches of the International Statute of Secrecy, such as the Ilfracombe Incident and the exposure of America's magical society caused by Dorcas Twelvetrees, also lead to Memory Charms being used on a massive scale. But even then, there's usually no guarantee that every Muggle's memory has been erased.