  Let X be a geodesic metric space. Gromov proved that there exists k>0 such
that if every sufficiently large triangle T satisfies the Rips condition with
constant k times pr(T), where pr(T) is the perimeter T, then X is hyperbolic.
We give an elementary proof of this fact, also giving an estimate for k. We
also show that if all the triangles T in X satisfy the Rips condition with
constant k times pr(T), then X is a real tree.
  Moreover, we point out how this characterization of hyperbolicity can be used
to improve a result by Bonk, and to provide an easy proof of the (well-known)
fact that X is hyperbolic if and only if every asymptotic cone of X is a real
tree.
