Sometimes students understand right away...

I will be away for a few days at a conference next week. This means I will miss one of my upper division labs, and I scheduled a "day off" in the other upper division lab. I almost never cancel class, but the lab falls midweek, missing Thanksgiving, Labor Day and Columbus Day, so it already has more meetings than any other lab in our department. Another reason is that the students in the two classes overlap considerably, and we have a lot due in both (labs, huge problem sets, an exam, etc.).

With that set up, enter one of my more industrious and polite students:

"Dr. Twice?"

"Yes?"

"Mary and I thought we would come in next week, you know, because we don't have lab, and finish the second part of this one. Then we could have a day off later."

"Um, well, if you come in, then I don't really get the afternoon off."

"Ah. Never mind."