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.