I avoided most of the technical talk when I discussed the Möbius strip the other day. You — well, a mathematician — could describe the strip as a non-orientable compact manifold with boundary. The boundary part is obvious. The non-orientable bit is easy enough to understand, when you remember that thing about it being a one-sided surface. Compact is an idea worth its own essay sometime. In this context it amounts to “there aren’t any gaps inside it”. Manifold, too, is worth an essay, and I wrote one in 2018 about it. Thanks for reading.