Hi, I have a question about identifying closed and open surfaces. Usually, when I see some exercises in the subject of the divergence theorem/flux integrals, I am not sure when the surface is open and needed to be closed or if it is already closed. I mean for example a cylinder that is truncated, or tetrahedron in the first octant(x,y,z>=0)... I have seen some exercises that also add the "cover" to close the surface and some that don't. How should I figure out what to do in a given exercise to help me understand when I should closed the surface or not. Thank you!