Although carbon nanotubes consist of honeycomb carbon, they have never been fabricated from graphene
directly. Here, it is shown by quantum molecular-dynamics simulations and classical continuum-elasticity
modeling, that graphene nanoribbons can, indeed, be transformed into carbon nanotubes by means of twisting. The
chiralities of the tubes thus fabricated can be not only predicted but also externally controlled. This twisting route
is an opportunity for nanofabrication, and is easily generalizable to ribbons made of other planar nanomaterials.