  The two-point correlation function has been the standard statistic for
quantifying how galaxies are clustered. The statistic uses the positions of
galaxies, but not their properties. Clustering as a function of galaxy
property, be it type, luminosity, color, etc., is usually studied by analysing
a subset of the full population, the galaxies in the subset chosen because they
have a similar range of properties. We explore an alternative
technique---marked correlations---in which one weights galaxies by some
property or `mark' when measuring clustering statistics. Marked correlations
are particularly well-suited to quantifying how the properties of galaxies
correlate with their environment. Therefore, measurements of marked statistics,
with luminosity, stellar mass, color, star-formation rate, etc. as the mark,
permit sensitive tests of galaxy formation models. We make measurements of such
marked statistics in semi-analytic galaxy formation models to illustrate their
utility. These measurements show that close pairs of galaxies are expected to
be red, to have larger stellar masses, and to have smaller star formation
rates. We also show that the simplest unbiased estimator of the particular
marked statistic we use extensively is very simple to measure---it does not
require construction of a random catalog---and provide an estimate of its
variance. Large wide-field surveys of the sky are revolutionizing our view of
galaxies and how they evolve. Our results indicate that application of marked
statistics to this high quantity of high-quality data will provide a wealth of
information about galaxy formation.
