180 Days of Ideas for Discussion in Math Class.
(as of 9July2014, we're in overtime!)
Slope. The hypotenuse of the red triangle has slope 3/8, and the slope of the cyan triangle has slope 2/5. Since 3/8 < 2/5, the "hypotenuse" of the upper figure bends inward, and the "hypotenuse" of the lower figure bends outward. The bending accounts for the difference in area between the two figures.