  Integral theorems such as Stokes' and Gauss' are fundamental in many parts of
Physics. For instance, Faraday's law allows computing the induced electric
current on a closed circuit in terms of the variation of the flux of a magnetic
field across the surface spanned by the circuit. The key point for applying
Stokes' theorem is that this surface must be orientable. Many students wonder
what happens to the flux through a surface when this is not orientable, as it
happens with a M\"obius strip. On an orientable surface one can compute the
flux of a solenoidal field using Stokes' theorem in terms of the circulation of
the vector potential of the field along the oriented boundary of the surface.
But this cannot be done if the surface is not orientable, though in principle
this quantity could be measured on a laboratory. For instance, checking the
induced electric current on a circuit along the boundary of a surface if the
field is a variable magnetic field. We shall see that the answer to this puzzle
is simple and the problem lies in the question rather than in the answer.
