"Of course," Severus said, arching one eyebrow with surprise. "Doing both the mapping and the attack on one night would create too many variables. Too many opportunities for things to go wrong. By separating the two, there is no time pressure on the mapping and we take our time, ensure that we are not seen."
He gave a small wryly amused look. "Yes, you will quickly find that there is a vast difference between theory and practice when it comes to this sort of thing. Plans are all well and good but they must be simple and they must be flexible to cope with random chance."
He paused for a moment and was about to continue when someone knocked tentatively at his door.
"Come in," he called as he straightened up and the door opened to reveal one of his Second Year Slytherins, his expression a mix of anger, nervousness and unhappiness. Severus restrained the sigh that was threatening and wondered just what his serpents had been up to now.
"Professor Cornfoot, I am afraid that we will have to continue this discussion at a later time," he said, gesturing for the Second Year to enter. He waited until Stephen had left and closed the door behind him then he waved the boy to the seat Stephen had vacated. "Now, what has happened?"