Zac's App
Cavalieri's Principle applies in many contexts, but this is the most fundamental.
This sketch was the idea of Jill Knaus (@JillKnaus) and goes with the start of a lesson plan for the Principle (Common Core HSG-GMD.A.2): http://bit.ly/1hiHIUI
The sketch asks:
What do you notice?
What can you change?
Compare & contrast the triangles and their measures.
What do you wonder about?
More GeoGebra at mathhombre.blogspot.com.