The Mori cone of curves of the Grothendieck-Knudsen moduli space of
stable rational curves with n markings, is conjecturally generated by
the one-dimensional strata (the so-called F-curves). A result of Keel
and McKernan states that a hypothetical counterexample must come from
rigid curves that intersect the interior. In this talk I will show
several ways of constructing rigid curves. In all the examples a
reduction mod p argument shows that the classes of the rigid curves that
we construct can be decomposed as sums of F-curves. This is joint work
with Jenia Tevelev.