  I study the physical nature of traces (or memories). Surprisingly, (i)
systems separation with (ii) temperature differences and (iii) long
thermalization times, are sufficient conditions to produce macroscopic traces.
Traces of the past are ubiquitous because these conditions are largely
satisfied in our universe. I quantify these thermodynamical conditions for
memory and derive an expression for the maximum amount of information stored in
such memories, as a function of the relevant thermodynamical parameters. This
mechanism transforms low entropy into available information.
