I will not tell Sir Cadogan that the Knights who say Ni have challenged him to a duel and then have students yell "Ni!" from various directions.
"Ready?" Harry asked, signaling to the rest of the DA, and a few others, that it was time to start. He looked over toward the entrance to the corridor leading to the Gryffindor portrait, and whistled loudly, attracting the attention of many students milling about. Most of them were in on Harry's plot, and though they didn't understand it, they agreed because, well, he's Harry fricken Potter that's why.
Harry watched as Colin Creevey ran around the corner, and up to the painting of Sir Cadogan, and leaned down with his palms on his knees panting as if he had raced three times around the pitch on foot.
"Mr. Sir Cadogan Sir!" He panted, being as theatrical as possible, knowing that the painting was eating it all up. "I've just gotten word that the student's have formed a group. The knight's who say Ni, and anyone who says Ni wants to challenge you to a duel!"
Sir Cadogan puffed up his chest and said loudly, "SCURVY BASTARDS! Come and meet your DOOM!"
Harry clapped his hands a few seconds later, and a chorus of 'Ni's!' rose up and met Sir Cadogan, 'challenging' him.
The knight, for his part yelled a battle cry and called those who said 'Ni!' to stop being cowards and face their DOOM.
For the rest of the day, the DA, and a few other students sent up a few 'Ni's!' whenever they were near the portrait of the knight.
