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 Define In[34]:= Clear[\[CapitalOmega]] \[CapitalOmega] = ImplicitRegion[2 <= (x^2 + y^2 + z^2) <= 4, {x, y, z}] Out[35]= ImplicitRegion[2 <= x^2 + y^2 + z^2 <= 4, {x, y, z}]  plot it to see the thickness of the shell In[36]:= RegionPlot3D[\[CapitalOmega], {x, -2, 2}, {y, -2, 2}, {z, -2,1/2}] During evaluation of In[36]:= RegionPlot3D::boolf: \[CapitalOmega] must be a Boolean function. >> Out[36]= RegionPlot3D[\[CapitalOmega], {x, -2, 2}, {y, -2, 2}, {z, -2, 1/2}]  to see nothing but an error message ... now RegionPlot3D[\[CapitalOmega], AspectRatio -> Automatic]  the good old sphere in 3D is shown and now \[CapitalOmega] is a Boolean function. The following works too In[38]:= Clear[\[CapitalOmega]] \[CapitalOmega] = ImplicitRegion[2 <= (x^2 + y^2 + z^2) <= 4, {{x, -2, 2}, {y, -2, 2}, {z, -2, 1/2}}] Out[39]= ImplicitRegion[2 <= x^2 + y^2 + z^2 <= 4 && -2 <= x <= 2 && -2 <= y <= 2 && -2 <= z <= 1/2, {x, y, z}] In[40]:= RegionPlot3D[\[CapitalOmega], AspectRatio -> Automatic]