Beautifully done!
[From Girl’s Angle: A Math Club for Girls, via Albany Area Math Circle.]
Do you know why this proof works? How can we be sure the red and yellow areas don’t change as they slide around?
Cavalieri’s Principle
For my math club students, I call it the “stack of cards” rule: If you have a stack of playing cards (or any other rigid shapes), the volume of the stack stays the same when you slant the stack different ways.
Here is an interactive demonstration of Cavalieri’s Principle (with very thick, 2-dimensional “playing cards”) from Jim Loy’s Mathematics Page.
Wikipedia has several examples of area and volume problems solved by Cavalieri’s Principle.
Denise Gaskins' Let's Play Math 
Sliding of triangles works because the apex glides along a line parallel to the base so neither the base nor the altitude change. For the same reason, the area of a parallelogram does not change if one of the sides slides along the line it is on. Here is a simplified version where the sliding shapes do not even change:
http://www.cut-the-knot.org/pythagoras/Pyth69PWW.shtml
And here is another demonstration of Cavalieri’s principle
http://www.cut-the-knot.org/Curriculum/Calculus/Cavalieri.shtml
Great example.
I want to do this using GeoGebra.