What happens when you have an unexpected absence?  In my classroom I have emergency plans set aside for times like these.  I teach chemistry and physics and my emergency plans typically consist of conversions or triangle trigonometry; two very useful skills for each of these classes. It turns out to just be busy work for them but keeps their skill set fresh.  I wish I could have some emergency plan in which students could continue their learning in an uninterrupted way.  Of course you could always just have them read from their book and answer questions or watch a movie but how productive is that?  Has anyone every found a set of emergency plans that were effective?  What kinds of things do you leave for you students when you have an unexpected absence?