  We present a novel scenario where a scalar field acquires a mass which
depends on the local matter density: the field is massive on Earth, where the
density is high, but is essentially free in the solar system, where the density
is low. All existing tests of gravity are satisfied. We predict that
near-future satellite experiments could measure an effective Newton's constant
in space different than that on Earth, as well as violations of the equivalence
principle stronger than currently allowed by laboratory experiments.
