About an hour later, the second-year boys' dormitory was almost quiet, and to an outside observer would have appeared deserted. Draco was downstairs, playing cards with some of the third-years, Vincent and Gregory still hadn't woken from the Sleeping Draught, and Theo had gone home for the holidays.

However, behind the curtains of Leo's bed, two thirteen-year-olds were sitting cross-legged, each holding a single vial of mud-coloured, bubbling liquid.

"Hair," said Leo softly, uncorking his vial.

Marisa reached up to her head, pulled off a single gold-coloured hair and dropped it into Leo's vial. Immediately the potion changed from the colour of mud to a deep black, tinged with gold flecks.

"Your turn," she said to Leo, who dropped one of his own dark hairs into her vial.

This time, the potion turned a pale grey: it didn't look particularly appetising, but it had a strange sort of shimmering quality to it.

"You drink first," said Leo.

"No, you first," replied Marisa.

"We'll do it together," Leo decided. "Ready – three, two, one – "

Marisa raised the vial to her lips, tilted her head back and gulped it down in one go. It tasted odd: there was an overwhelming sharpness to it, stronger than even a lime, which made her blink furiously, but before she could even register that she felt a horrible pain; she was certain she would be sick – her entire body was burning – then she felt almost as if she was bubbling and melting.

As she clutched her knees, gasping, she felt her body changing: she saw her fingers becoming slightly thicker, her skin becoming just a touch paler – she was shooting upwards, unnaturally tall – her legs were becoming longer.

Then it stopped. She blinked as she saw that she was sitting opposite herself. Leo looked exactly like her. It felt unbelievably weird looking down on herself from just a little higher.

"This feels so weird," she said – but it was Leo's crisp, precise voice that came out of her mouth.

"We should probably swap clothes," said Leo in Marisa's own voice, practical as ever. "People are going to notice something when they see a boy in skirt and tights and a girl in trousers."

The Hogwarts uniforms were slightly different for boys and girls.

Marisa nodded. "No looking at my underwear," she said as she pulled off her skirt and tights. "Your legs are really hairy!" She could imagine the look on Leo's face without having to look at him, and then realised that it was actually her own face.

She caught the pair of smart black trousers that Leo tossed her. Her first reaction was to think that they would be too big for her, but then she realised that Leo's body was a lot bigger than hers and in fact they would fit perfectly, so she pulled them on.

"Shall we go… Marisa?" she asked, already putting on her act for the benefit of the others.

"Certainly, Leo," replied Leo, smiling slightly, and the pair walked out of the dormitory and downstairs to the common room.