What is #Rigor?

Last month, we focused on student motivation.  This month, let's turn our attention to rigor.  What exactly is rigor?  If you've read any of my books, you know it's "creating an environment in which each student is expected to learn at high levels, each student is supported so he or she learns at high levels, and each student demonstrates learning at high levels (2008)."  I was recently in Killeen, Texas working with teachers.  Annually Masker, at Hay Branch Elementary School, came up with her own:

Rigor is the resistance you encounter that will strengthen your understanding.  Example:  like when a runner trains to run on sand.

Isn't that a great visual example?  I love it!!