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Select all the surfaces and press ESC twice. GiD automatically generates the volume of the cube. The volume viewed on the screen is represented by a cube with an interior color of sky blue. 
Before proceeding with the mesh generation of the volume, we should eliminate the information of the structured mesh created previously for the surface. Do this by selecting Mesh->Reset mesh data, and the following dialog box will appear on the screen:
In which the user is asked to confirm the erasure of the mesh information.
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