When teaching a Library Skills class, ensure you have a whiteboard marker handy.
Do not, under any circumstances, use the miscellaneous marker that is left in the whiteboard's vicinity without checking what kind of marker it is.
Failure to follow this instruction may lead to the use of a permanent marker to draw a Venn diagram on the whiteboard, and to illustrate truncation.
A failure of this magnitude will leave you cleaning the whiteboard for half the time that the class went for. Of course, if you were planning on going to the gym, this counts as exercise so you can skip it and have another Caramello Koala instead.