Fred and George pulled on their cloaks, grabbed their wands and other materials they would need, and headed off to the Potion Master's classroom. This would either be their greatest triumph, or the downfall of their pranking.
"Alohamora," Fred whispered, unlocking the door to Snape's classroom. The two twins started to walk inside but George put his arm out to stop his brother from going any farther.
"Wait!" he said. He quickly cast a silencing charm on them. They could hear themselves and each other, but nobody else could hear them. "Now we won't wake Snape up with our talking."
"Should we risk entering his private quarters to put a sleeping charm on him?" Fred asked.
George pondered this. "Hmm… That would be pretty risky."
"And what we're doing's not?" Fred retorted.
"Whatever." George snapped. "If he wakes up… Well… We'll cross that bridge when we get there."
Fred shrugged his shoulders in response, and set the materials down on a table. "Okay, where should we start?"
"Well, I think we should start with the potions. He said that the seventh years would be learning how to create a fever-reducing potion today. Do you have the potions book with the ingredients in it?" George asked.
Fred picked up the book, and read the ingredients aloud to his brother.
"Perfect. He has all the ingredients next to each other on his shelf. Hand me the string, Fred." George instructed his brother.
George tied the string around each jar containing the ingredients, weaving it through the different jars, so the string connected them to each other. He then cast a concealment charm on the string so Snape couldn't see it.
"Brilliant," said Fred. "What's next?"
"The cooling charm on his cauldron." George told Fred. "No matter how hot he makes the fire, the potion will never become the right temperature and won't brew properly."
As George was casting the cooling charm on the cauldron, Fred was preparing the next step: applying a sticking charm on Snape's quill. When Snape would pick up the quill, it would stick to his hand and wouldn't come off.
The next step, and George's personal favorite, was the one that would be the funniest to witness. They were going to change Snape's appearance to make him look like a spitting image of Professor McGonagall.
Fred and George charmed the doorframe that led into Snape's classroom with a charm that worked like the Pollyjuice Potion. When Snape, who always arrived before his students, walked through the doorframe, the charm would stick to him and over twenty minutes, would make him look like Professor McGonagall.
The final step was to tie a rope around one of the desk chairs and attach it to his desk (using a sticking charm) so that when he would walk in front of the class he would trip and make a fool of himself. The rope was hidden by a concealment charm so nobody could see it.
The twins stood at the back of the classroom, admiring their hard work.
"Well, I believe that this will go either really well," George started.
"Or really bad." Fred finished.
The two walked out of the classroom, re-locked the door, and walked back to Gryffindor Tower, canceling the silencing charm on the way. Oh, what a surprise Snape was in for. Little did he know that during his first class of the day, seventh year potions with the Gryffindors and Slytherins, he would fall victim to one of the Weasley twin's best thought out pranks.