A sphere is inscribed in a cube. What is the ratio of the volume of the inscribed sphere to the volume of the cube? Express your answer as a common fraction in terms of $\pi$.
volume of the sphere = 4/3 pi r3
Volume of cube = 2r x 2r x 2r = 8r^3 ratio 4/3 pi r^3 / 8r^3 = pi/6