Leo and Marisa both signed up to stay at Hogwarts over the Christmas holidays: Leo because Lucius wanted him and Draco to keep an eye on the situation with the Chamber and Marisa because she had decided that Hogwarts was far, far more exciting than home.
In Potions, one Thursday, a few days after Leo and Marisa's birthdays, the Gryffindors and Slytherins were brewing the Swelling Solution. Leo's potion was slightly too watery, even though he'd added exactly the amount the instructions called for. Marisa's, meanwhile, was just the right consistency.
"How are you doing that?" asked Leo. "I added just as much water as you did, why is mine so watery?"
Marisa glanced around to make sure Professor Snape wasn't watching: he was sneering at Potter's potion and not paying them any attention. Draco and Pansy were laughing at whatever he was saying.
"I added the water a little bit at a time," replied Marisa, "and stirred it in gradually until the consistency was right."
"That's not what the instructions say… though it does make sense, and it certainly seems to have worked for you. I'll bear that in mind for future reference."
They each returned to their work, concentrating on the fires they were lighting underneath their cauldrons to get them at precisely the right temperature.
As they were doing that, they heard a loud splashing sound and suddenly the greenish liquid in one of the cauldrons flew into the air and covered the students. Marisa got a bit on her hand, which immediately began to increase in size. Not knowing quite what to do, she stared at it as it inflated to the size of a dinner plate.
For a few moments, the room was filled with chaos, before Professor Snape yelled "SILENCE! Anyone who has been splashed, come here for a Deflating Draught – when I find out who did this – "
Marisa staggered up to Professor Snape's desk, struggling to support the weight of her massive arm, and as she did so she saw Granger slipping out of Snape's office, robes bulging, and then she immediately understood: ingredients for the Polyjuice Potion.
She shared a brief glance at Leo, who was still standing by his cauldron, and knew that he had seen, too, and he also knew exactly what was going on.
This information would be extremely useful in the future. She smiled slowly.
