  We find a (quasi-)local first law of thermodynamics, $\Delta E = T \Delta S -
W$, connecting gravitational entropy, $S$, with matter energy and work. For
Einstein gravity $S$ is the Bekenstein-Hawking entropy, while for general
theories of gravity $S$ is the Wald entropy, evaluated on the stretched future
light cone of any point in an arbitrary spacetime, not necessarily containing a
black hole. The equation can be written as $\rho \Delta V = T \Delta S - p
\Delta V$ by regarding the energy-momentum tensor as that of a fluid.
