He understood, of course. He even knew more: the Time-Turner had been developed using the Hour-Reversal Charm.
The Hour-Reversal Charm was dangerous and unstable. It could turn back time by up to five hours. According to Professor Saul Croaker, the charm had been incorporated into the Time-Turner because containing an unstable spell in a magical vessel made it work better. Each grain of sand in its hourglass could be thought of as an Hour-Reversal Charm of a different strength.
Will continued, "Of course, Professor McGonagall. I read about how dangerous Time-Turners are. Eloise Mintumble was an Unspeakable in the Department of Mysteries. During an experiment, she used a Time-Turner to travel into the past, but because she went back more than a few hours, she died in 1899.
"Eloise Mintumble traveled from 1899 to 1402 and was trapped there for five days. She changed the course of history: twenty-five descendants of people she met in the past were never born.
"Although Eloise was trapped in 1402 for five days, when she finally returned to the present, her body had aged five centuries. She suffered irreparable damage and eventually died at St Mungo's Hospital for Magical Maladies and Injuries.
"Her journey changed the lives of everyone she encountered in the past. At least twenty-five descendants vanished from the present, as though they had never been born. There were also worrying signs that time itself had been disturbed. After she returned, Tuesday lasted two and a half days, while Thursday lasted only four hours.
"That shows time in our world can bend and change. It can be tampered with. We have to treat it with extreme care."
Professor McGonagall looked pleased. "You're as knowledgeable as ever, Behrens."
"I wouldn't dare call myself knowledgeable in front of you. I just wanted to learn