There are many reasons to modify Einstein's theory of gravity, general relativity. Scalar-tensor gravity is the prototypical modification and can be described as an effective fluid by writing its field equations as effective Einstein equations with an effective “fluid” as a source. This fluid is dissipative and, almost miraculously, satisfies the constitutive laws of Eckart's thermodynamics. This leads to a thermal picture with a “temperature of gravity”  and general relativity as the T=0 equilibrium state. Intriguing consequences are: gravity may or may not approach its equilibrium state; singularities are “hot” (deviate from relativity); the expansion of space “cools” gravity; degenerate theories with non-dynamical scalar field are also states of equilibrium; standard scalar fields in relativity and Einstein frame scalar-tensor gravity (an alternative description) correspond to T=0 but nonzero chemical potential, correcting the standard lore.