  In this paper we prove some results on the boundary behavior of solutions to
fractional elliptic problems. Firstly, we establish a Hopf Lemma for solutions
to some integro-differential equations. The main novelty of our result is that
we do not assume any global condition on the sign of the solutions. Secondly,
we show that non-trivial radial solutions cannot have infinitely many zeros
accumulating at the boundary.
  We provide concrete examples to show that the results obtained are sharp.
