Fred sent George a grin as they snuck into the dungeons. Everyone was at lunch, and Snape didn't have another potions lesson until the next morning.
It was just perfect.
Casting a quick detection charge, which told them that there wasn't anyone else in the dungeons, they slipped into the potions laboratory.
The Slytherin section of the lab hadn't changed in all the years they had been in Hogwarts. The Slytherins always sat on the same side, leaving whichever other house, usually the Gryffindors, to sit on the other side of the classroom.
Which meant they couldn't possibly fail this time (last time they had set it up, hoping that Snape would actually sit down, and he didn't) because the Slytherins couldn't just not attend a potions lesson. Especially since they enjoyed the favouritism they found there.
Fred sent George a thumbs up, and George dug into his bag for the portable swamp they had been working on. It wasn't perfect yet, but these were a bunch of Slytherins so that was okay.
Written for the September Back to School Challenge: rucksack/bag