He closed the door behind them and sealed it with a Colloportus Charm.

“What was it you wanted to show me, Albus?” she asked with a wry grin.

He said nothing but withdrew his wand and pointed it at her. A moment later, she found herself wearing her old school uniform, complete with Gryffindor tie and Head Girl badge.

“Albus, I swear—” she began, but he cut her off.

“Enough, Miss McGonagall. Please take your seat.”

As she turned, she felt her hair released from its bun to cascade loosely down her back.

She took a seat at her old desk in the front row. As she sat, and her skirt rode up against her a bit, she realised she had on no knickers. She looked up to find that Albus wore an old-fashioned teaching robe.

“Miss McGonagall,” he said, “perhaps you would be so good as to give me the five principal exceptions to Gamp’s Law of Elemental Transfiguration.”

“Of course, Professor,” she said, getting into the spirit of the game. “One: conjured foodstuffs always Vanish in the presence of digestive enzymes. Two: carbon-based life forms cannot be conjured, nor can they be reanimated after death. Three: water cannot be conjured in the absence of greater than point five five parts per million atmospheric hydrogen. Four: fire can only be conjured in the presence of an oxidising agent in sufficient quantity to produce rapid oxidisation of a combustible material. Five: transition metals, notably gold and platinum, cannot be conjured.”

“Correct. But I must ask, Miss McGonagall, why you are not wearing any knickers.”