Additionally you can do it with Plot3D:
Plot3D[{Sqrt[1 - x^2 - y^2], Sqrt[.25 - x^2 - y^2]}, {x, -1, 1}, {y, -1, 1}, Mesh -> False]
and RevolutionPlot3D:
Show[
RevolutionPlot3D[{Sin[t], Cos[t]}, {t, 0, Pi}, {s, 0, Pi}, Mesh -> False],
RevolutionPlot3D[{.5 Sin[t], .5 Cos[t]}, {t, 0, Pi}, {s, 0, Pi}, Mesh -> False]
]
