  We introduce a $1$-player game called \emph{pin the loop} and analyse its
complexity. A loop is a generic immersion of the circle in a surface,
considered up to isotopy. A loop is taut when it is minimally intersecting in
its homotopy class. A pinning set of a loop is a set of points $P$ in the
surface avoiding the loop, such that the loop is taut in the surface punctured
at $P$. The pinning number is the minimal cardinal of its pinning sets, and a
pinning set is optimal if it has that cardinal.
  We show that the decision problem associated to computing the pinning number
of a plane loop is \texttt{NP-complete}. We implement a polynomial algorithm to
check if a given point-set is pinning, adapting a method of Birman--Series for
computing intersection numbers of curves in surfaces. After improving a theorem
of Hass--Scott characterising taut loops in surfaces, we reduce the problem to
a boolean formula whose solutions correspond to the pinning sets. Finally, we
reduce the vertex-cover problem for graphs to an instance of the pinning
problem for plane loops.
  Our results imply that given a closed geodesic in a complete Riemannian
surface with punctures, it is hard to guess the location of the punctures from
the sole isotopy class of the geodesic in the compactified surface.
